Merge FieldMapper and MappedFieldType

[jpountz]

MappedFieldType had originally been split out of FieldMapper in order to work around problems that having multiple types in the same index introduced. Now that we introduced the guarantee that every index has at most one type, we could merge MappedFieldType and FieldMapper together.

[elasticmachine]

Pinging @elastic/es-search

[jpountz]

Both @javanna and @romseygeek are exploring features that wouldn't require doing anything at index time, they would only leverage MappedFieldType for search-time logic, so maybe we shouldn't merge actually?

[romseygeek]

I think we could still do some re-organising here. For example, FieldMapper has the parseCreateField method on it which is used for indexing, while MappedFieldType has all the various xxxQuery methods on it, for searching - I think it might make sense for those to be the other way round?

[jtibshirani]

A note that flattened fields currently take advantage of the FieldMapper/ MappedFieldType distinction. We create a new MappedFieldType on-the-fly inside FieldTypeLookup to support searches on the subfields. This new field type doesn't correspond to its own field in the mappings.

[romseygeek]

I opened a separate issue for the re-organisation: #56814

I think we can close this one though?

[jtibshirani]

Closing in favor of #56814, feel free to reopen if you disagree!