ESQL: allow returning an empty response if ignore_unavailable is used for non-existent indices

[astefan]

Description

Following #106636 merge ESQL queries touching inexistent indices from nonexistent1, nonexistent2 options "ignore_unavailable"="true" will still return "index not found" error while the same ES query POST /localhost:9200/nonexistent1,nonexistent2/_search?ignore_unavailable=true will not return an error.

We should look into having the same behavior as the one of ES and _search endpoint.

[elasticsearchmachine]

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

[astefan]

ESQL uses IndexResolver.FIELD_CAPS_INDICES_OPTIONS that has some small differences from _search default indices options:

• .concreteTargetOptions(IndicesOptions.ConcreteTargetOptions.ALLOW_UNAVAILABLE_TARGETS) meaning ignore_unavailable is true, while in SearchRequest the default is false
• allowClosedIndices(true) while for _search is false
• _search also has an extra .allowFailureIndices(true), which is newly added in 8.14. Failure indices seem to refer to: Add ability to create a data stream failure store #99134 (added in 8.12)

These differences mean that, when the option (ignore_unavailable or allow_no_indices) is not explicitly used (and the default takes precedence), then the behavior of ESQL differs from the one of _search. We either document these differences and make sure users are aware of them or we start using the _search default indices options and adapt EsqlIndexResolver to react accordingly to an empty _field_caps response.
Somewhat related: #107031