Multi-fields information in field caps API

[mayya-sharipova]

Display in fields caps information that fields belong together as multi-fields.

This could be useful:

• to present a user only a single field from multi-fields
• to make a decision for solutions which field to choose for certain types of queries

Relates to #49028

[elasticmachine]

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

[javanna]

With the addition of filters to field_caps (see #83636), consumers should be able to exclude multi_fields from the fields returned by field_caps. We went with this approach instead of enhancing the output of the field_caps API. Hopefully this addresses this issue although it does not implement exactly what was initially described.