Support the search 'fields' option in 'inner_hits'.

[jtibshirani]

Other field retrieval options like _source and docvalues_fields are available in the inner_hits block. We should also support the search fields option added in #60100.

This is a spin-off of the meta issue #60985.

[elasticmachine]

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