Painless: Create API to determine available languages for a context

[stu-elastic]

Ref: elastic/kibana#11959

Kibana needs to know which languages are available for a given context. That used to be available by calling: http://localhost:9200/_cluster/settings?include_defaults=true&filter_path=**.script.engine.*.inline

A couple of wrinkles:

1. Determining whether scripts are available as stored and/or inline is a per-node setting, so hitting different nodes may return different results.
2. The script contexts supported per ScriptEngine is not defined in a current API. For this we may add a method to ScriptEngine which returns supported contexts. If we add a method to the ScriptEngine interface, it's a breaking change.
3. Different contexts maybe be available for OSS and Basic licenses.

cc: @rjernst @jdconrad

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Scripting)