Move allocation awareness attributes to list setting

[ywelsch]

Allows the setting to be specified using proper array syntax, for example:

"cluster.routing.allocation.awareness.attributes": [ "foo", "bar", "baz" ]

Closes #30617

[elasticmachine]

Pinging @elastic/es-distributed

[bleskes]

LGTM. I wonder if this should be marked as breaking as I believe we now return this setting as a list where before we returned it as a string all the time.

[ywelsch]

@bleskes fortunately not. The setting is returned in the same way as it was set.

For example:

$ cat bla.json 
{
  "persistent": {
    "cluster.routing.allocation.awareness.attributes": "foo,bar"
  }
}

$ http PUT :9200/_cluster/settings < bla.json
HTTP/1.1 200 OK
content-encoding: gzip
content-length: 118
content-type: application/json; charset=UTF-8

{
    "acknowledged": true,
    "persistent": {
        "cluster": {
            "routing": {
                "allocation": {
                    "awareness": {
                        "attributes": "foo,bar"
                    }
                }
            }
        }
    },
    "transient": {}
}

$ http :9200/_cluster/settings
HTTP/1.1 200 OK
content-encoding: gzip
content-length: 105
content-type: application/json; charset=UTF-8

{
    "persistent": {
        "cluster": {
            "routing": {
                "allocation": {
                    "awareness": {
                        "attributes": "foo,bar"
                    }
                }
            }
        }
    },
    "transient": {}
}

versus

$ cat bla.json 
{
  "persistent": {
    "cluster.routing.allocation.awareness.attributes": ["foo", "bar"]
  }
}

$ http PUT :9200/_cluster/settings < bla.json
HTTP/1.1 200 OK
content-encoding: gzip
content-length: 123
content-type: application/json; charset=UTF-8

{
    "acknowledged": true,
    "persistent": {
        "cluster": {
            "routing": {
                "allocation": {
                    "awareness": {
                        "attributes": [
                            "foo",
                            "bar"
                        ]
                    }
                }
            }
        }
    },
    "transient": {}
}

$ http :9200/_cluster/settings
HTTP/1.1 200 OK
content-encoding: gzip
content-length: 110
content-type: application/json; charset=UTF-8

{
    "persistent": {
        "cluster": {
            "routing": {
                "allocation": {
                    "awareness": {
                        "attributes": [
                            "foo",
                            "bar"
                        ]
                    }
                }
            }
        }
    },
    "transient": {}
}

[bleskes]

@ywelsch cool. sometimes you get lucky :)