ES|QL: Allow simplified full text search using text in where clause

[timgrein]

Summary

This PR adds a simplified way of doing a full-text-search over all text fields in an index using ES|QL.

The new syntax looks like the following:

FROM idx | WHERE "query string"

instead of:

FROM idx | WHERE MULTI_MATCH("query string", field_one, ..., field_n)

This functionality inherits the underlying restrictions of multi_match (not working with semantic_text out of the box). I've intentionally decided against handling edge cases like raising errors, when multiple text fields, which include one or more semantic_text fields, are queried as this should probably be the responsibility of the multi_match function itself. It would also be not too straightforward as the information, if a text field is of type semantic_text is not known in the analysis/verification phase, but only during physical optimization AFAIU. It felt wrong to hack this into earlier phases of query processing, but happy to discuss more - could've missed something of course :)

Querying a single semantic_text field works as the query will be translated to use MATCH instead of MULTI_MATCH, if only one text (including semantic_text) field is present in the queried index.

Requests for testing:

Create index with multiple text fields:

curl --location --request PUT 'http://localhost:9200/example_full_text_index_multiple_fields' \
--header 'Content-Type: application/json' \

--data '{
    "mappings": {
        "properties": {
            "field_one": {
                "type": "text"
            },
            "field_two": {
                "type": "text"
            },
            "field_three": {
                "type": "long"
            }
        }
    }
}'

Indexing sample data:

curl --location 'http://localhost:9200/_bulk' \
--header 'Content-Type: application/json' \
--data '{ "index": { "_index": "example_full_text_index_multiple_fields", "_id": "1" } }
{ "field_one": "Star Wars" }
{ "index": { "_index": "example_full_text_index_multiple_fields", "_id": "2" } }
{ "field_one": "Lord of the Rings" }
{ "index": { "_index": "example_full_text_index_multiple_fields", "_id": "3" } }
{ "field_one": "Harry Potter" }
{ "index": { "_index": "example_full_text_index_multiple_fields", "_id": "4" } }
{ "field_one": "Pulp Fiction", "field_two": "Star Trek" }
'

Normal multi-match ES|QL search:

curl --location 'http://localhost:9200/_query?format=txt' \
--header 'Content-Type: application/json' \
--data '{
    "query": "FROM example_full_text_index_multiple_fields | WHERE MULTI_MATCH(\"Star\", field_one, field_two)"
}'

New simplified ES|QL full-text-search:

curl --location 'http://localhost:9200/_query?format=txt' \
--header 'Content-Type: application/json' \
--data '{
    "query": "FROM example_full_text_index_multiple_fields | WHERE \"Star\""
}'

[timgrein]

Alternatively we could check, if MultiMatch contains one field, which is an instance of UnresolvedStar instead of introducing a separate "marker class" FullTextSearch.

[timgrein]

Will add a test making sure options are passed correctly from MultiMatch to Match