Allow MappedFieldType impls to hide themselves from field caps

[romseygeek]

Text fields can optionally index their content in multiple ways to speed up
phrase or prefix queries, implemented via named subfields with specialised
analysis chains. As these subfields need to be searchable, they have their
own FieldMappers and MappedFieldTypes; unfortunately, this means that
they can end up being returned in calls to the field_caps endpoint, where
really they are implementation details and should be hidden from users.

This commit adds a fieldCaps() method directly to MappedFieldType,
called by the field capabilities action. Field types that should be hidden
from field_caps can return null from this method, and will be excluded
from the response.

Fixes #63446

[elasticmachine]

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

[jtibshirani]

Unfortunately this doesn't address my general concern behind #63446. These fields will still be exposed through FieldTypeLookup, so any field pattern in a search will match them. For example fetching fields through "fields": ["*"] will match against these sub-fields as well, which is surprising behavior. It also feels off that we allow searching against them when they're not meant to be searched directly?

[romseygeek]

OK, I think I understand the concerns better now. The issue here is that all FieldMappers expose a MappedFieldType to the lookup, and this is specifically used to build the general index-time analyzer. So ideally, we'd want to build these implementation fields without registering FieldMappers or MappedFieldTypes, but somehow still register their analyzers.