ESQL: API endpoint (or similar) to retrieve available language versions

[alex-spies]

Relates #104890

For users connecting to an ES cluster, it's useful to know which ES|QL language versions can be used to communicate with the cluster (esp. in language clients and other forms of programmatic useage).

We could add e.g. an endpoint _query/version, which would return a JSON object like so:

{
    "versions": ["2024.01.01", "2024.09.01"]
}

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[stratoula]

Fantastic! Can the versions be ordered to find easily which is the latest?

[alex-spies]

This is not relevant anymore, as we dropped the version field in #109376.