Introduce a constant_keyword field. (#49713)

[jpountz]

This field is a specialization of the keyword field for the case when all
documents have the same value. It typically performs more efficiently than
keywords at query time by figuring out whether all or none of the documents
match at rewrite time, like term queries on _index.

The name is up for discussion. I liked including keyword in it, so that we
still have room for a singleton_numeric in the future. However I'm unsure
whether to call it singleton, constant or something else, any opinions?

For this field there is a choice between

1. accepting values in _source when they are equal to the value configured
   in mappings, but rejecting mapping updates
2. rejecting values in _source but then allowing updates to the value that
   is configured in the mapping
   This commit implements option 1, so that it is possible to reindex from/to an
   index that has the field mapped as a keyword with no changes to the source.

Backport of #49713

[elasticmachine]

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