Follow-up improvements to field retrieval.

[jtibshirani]

In #55363 we designed a first version of a fields option for retrieving document fields during a search. This issue tracks important follow-up changes to 'complete' the feature.

Changes that could affect the API (should be resolved before removing 'beta' flag):

• Support unmapped fields. Support unmapped fields in search 'fields' option. #63690
• Finalize how to return nested content. Finalize how search 'fields' option returns nested data. #63709
• Finalize how to handle flattened fields.
• Consider a limit on the number of fields retrieved. Investigate current limit stemming from our use of automata for include_unmapped. Should we limit the number of fields that can be retrieved by the “fields” API #69983

Other changes:

• Handle meta fields like _size. Handle metadata fields in search 'fields' option #63569
• Avoid returning hidden text fields like _index_prefixes. Text fields should not expose internal field types #63446
• Support the API in inner_hits. Support the search 'fields' option in 'inner_hits'. #61949
• Move FieldMapper#valueFetcher to MappedFieldType. Move FieldMapper#valueFetcher to MappedFieldType #62974

[elasticmachine]

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