Add more contexts to painless execute api (#30511)

This change adds two contexts the execute scripts against:

* SEARCH_SCRIPT: Allows to run scripts in a search script context.
This context is used in `function_score` query's script function,
script fields, script sorting and `terms_set` query.

* FILTER_SCRIPT: Allows to run scripts in a filter script context.
This context is used in the `script` query.

In both contexts a index name needs to be specified and a sample document.
The document is needed to create an in-memory index that the script can
access via the `doc[...]` and other notations. The index name is needed
because a mapping is needed to index the document.

Examples:

```
POST /_scripts/painless/_execute
{
  "script": {
    "source": "doc['field'].value.length()"
  },
  "context" : {
    "search_script": {
      "document": {
        "field": "four"
      },
      "index": "my-index"
    }
  }
}
```

Returns:

```
{
  "result": 4
}
```

POST /_scripts/painless/_execute
{
  "script": {
    "source": "doc['field'].value.length() <= params.max_length",
    "params": {
      "max_length": 4
    }
  },
  "context" : {
    "filter_script": {
      "document": {
        "field": "four"
      },
      "index": "my-index"
    }
  }
}

Returns:

```
{
  "result": true
}
```

Also changed PainlessExecuteAction.TransportAction to use TransportSingleShardAction
instead of HandledAction, because now in case score or filter contexts are used
the request needs to be redirected to a node that has an active IndexService
for the index being referenced (a node with a shard copy for that index).

Author: martijnvg

docs/painless/painless-execute-script.asciidoc

@@ -9,23 +9,24 @@ The Painless execute API allows an arbitrary script to be executed and a result
 .Parameters
 [options="header"]
 |======
-| Name             | Required  | Default                | Description
-| `script`         | yes       | -                      | The script to execute
-| `context`        | no        | `painless_test`        | The context the script should be executed in.
+| Name              | Required  | Default                | Description
+| `script`          | yes       | -                      | The script to execute
+| `context`         | no        | `painless_test`        | The context the script should be executed in.
+| `context_setup`   | no        | -                      | Additional parameters to the context.
 |======
 
 ==== Contexts
 
 Contexts control how scripts are executed, what variables are available at runtime and what the return type is.
 
-===== Painless test script context
+===== Painless test context
 
 The `painless_test` context executes scripts as is and do not add any special parameters.
 The only variable that is available is `params`, which can be used to access user defined values.
 The result of the script is always converted to a string.
 If no context is specified then this context is used by default.
 
-==== Example
+====== Example
 
 Request:
 
@@ -52,4 +53,124 @@ Response:
   "result": "0.1"
 }
 --------------------------------------------------
-// TESTRESPONSE
+// TESTRESPONSE
+
+===== Filter context
+
+The `filter` context executes scripts as if they were executed inside a `script` query.
+For testing purposes a document must be provided that will be indexed temporarily in-memory and
+is accessible to the script being tested. Because of this the _source, stored fields and doc values
+are available in the script being tested.
+
+The following parameters may be specified in `context_setup` for a filter context:
+
+document:: Contains the document that will be temporarily indexed in-memory and is accessible from the script.
+index:: The name of an index containing a mapping that is compatable with the document being indexed.
+
+====== Example
+
+[source,js]
+----------------------------------------------------------------
+PUT /my-index
+{
+  "mappings": {
+    "_doc": {
+      "properties": {
+        "field": {
+          "type": "keyword"
+        }
+      }
+    }
+  }
+}
+
+POST /_scripts/painless/_execute
+{
+  "script": {
+    "source": "doc['field'].value.length() <= params.max_length",
+    "params": {
+      "max_length": 4
+    }
+  },
+  "context": "filter",
+  "context_setup": {
+    "index": "my-index",
+    "document": {
+      "field": "four"
+    }
+  }
+}
+----------------------------------------------------------------
+// CONSOLE
+
+Response:
+
+[source,js]
+--------------------------------------------------
+{
+  "result": true
+}
+--------------------------------------------------
+// TESTRESPONSE
+
+
+===== Score context
+
+The `score` context executes scripts as if they were executed inside a `script_score` function in
+`function_score` query.
+
+The following parameters may be specified in `context_setup` for a score context:
+
+document:: Contains the document that will be temporarily indexed in-memory and is accessible from the script.
+index:: The name of an index containing a mapping that is compatable with the document being indexed.
+query:: If `_score` is used in the script then a query can specified that will be used to compute a score.
+
+====== Example
+
+[source,js]
+----------------------------------------------------------------
+PUT /my-index
+{
+  "mappings": {
+    "_doc": {
+      "properties": {
+        "field": {
+          "type": "keyword"
+        },
+        "rank": {
+          "type": "long"
+        }
+      }
+    }
+  }
+}
+
+
+POST /_scripts/painless/_execute
+{
+  "script": {
+    "source": "doc['rank'].value / params.max_rank",
+    "params": {
+      "max_rank": 5.0
+    }
+  },
+  "context": "score",
+  "context_setup": {
+    "index": "my-index",
+    "document": {
+      "rank": 4
+    }
+  }
+}
+----------------------------------------------------------------
+// CONSOLE
+
+Response:
+
+[source,js]
+--------------------------------------------------
+{
+  "result": 0.8
+}
+--------------------------------------------------
+// TESTRESPONSE