Support semantic_text in ES|QL

[ChrisHegarty]

This meta issue tracks adding support for semantic_text to ES|QL.

• [x] #113920
• [ ] #115166
• [x] #115243
• [ ] Add support for semantic_text in the match function - we will need to issue requests to the the inference service to get the embeddings for the query string on the coordinator and then push down the match function as a semantic query when semantic_text fields are used https://github.com/elastic/elasticsearch/pull/116253
• [x] Enhance support for semantic_text such that it has the same support as keyword and text which are simply seen as string fields: https://github.com/elastic/elasticsearch/blob/dcbbbabd3d1b78bd4658ab69e85e7c55bcc8fced/x-pack/plugin/esql-core/src/main/java/org/elasticsearch/xpack/esql/core/type/DataType.java#L356-L358 With this change we will be able to use semantic_text fields in the same way we use keyword and text and be able to pass semantic_text field to any function/operator that can accept string fields.
  • support for functions/operators #115243
  • support for all commands https://github.com/elastic/elasticsearch/pull/117337
• [ ] bug fix: semantic_text fields are returned as nulls when using copy_to
• [ ] Full support in Kibana (TBD what this entails)
• [ ] Finally, once the previous items are done: enable feature flag outside of snapshot builds and change documentation to reflect the support for semantic_text

Follow-ups:

• [ ] use MatchQueryBuilder when match query supports semantic_text (relates to SEARCH-688)
  • [ ] if possible can we revert the build.gradle changes from https://github.com/elastic/elasticsearch/pull/116253 ? - maybe x-pack-esql-core does not need x-pack-ml anymore.
• [ ] unmute serverless test for org.elasticsearch.xpack.esql.qa.mixed.EsqlClientYamlIT test {p0=esql/40_unsupported_types/semantic_text declared in mapping}

This work will take a while, but can be paralleled once the initial step is merged, thus unblock adding support for semantic_text in the match function.

[elasticsearchmachine]

Pinging @elastic/es-search (Team:Search)

[elasticsearchmachine]

Pinging @elastic/kibana-esql (ES|QL-ui)

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)