This implements the `fields` API in `_search` for runtime fields using
doc values. Most of that implementation is stolen from the
`docvalue_fields` fetch sub-phase, just moved into the same API that the
`fields` API uses. At this point the `docvalue_fields` fetch phase looks
like a special case of the `fields` API.
While I was at it I moved the "which doc values sub-implementation
should I use for fetching?" question from a bunch of `instanceof`s to a
method on `LeafFieldData` so we can be much more flexible with what is
returned and we're not forced to extend certain classes just to make the
fetch phase happy.
Relates to #59332
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