Add `_terms_enum` support for the `version` field type

[cbuescher]

Description

The _terms_enum API currently supports keyword, constant_keyword and
flattened fields. We had some recent discussions around whether the version field type should also be supported since it is intended as a specialization of the keyword field for handling software version values. I'm opening this issue for further discussion.

[elasticmachine]

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