Add include_type_name in indices.exitst REST API spec

[cbuescher]

The parameter exists in the indices.get API already, they both map to the same
REST action. Although the response body for the HEAD request used for checking
existence is empty, the parameter can be used to avoid deprecation warnings and
should be added so clients that generate their API from the rest spec pick it
up.

Closes #43905

[elasticmachine]

Pinging @elastic/es-search

[jtibshirani]

@cbuescher thanks for tackling this bug! My sense is that we should actually remove the deprecation warning for indices.exist requests, since it has no possibility of returning a type. In particular, we could check if request.method().equals(HEAD) and avoid emitting a deprecating warning in that case. I think it might be confusing for a user to have to add include_type_name to a request where there are no types in the request/ response.

[cbuescher]

we should actually remove the deprecation warning for indices.exist requests

Thanks for the feedback, that was another solution I briefly thought about. Will update the PR accordingly.

[cbuescher]

@jtibshirani I opened #43963 in stead of this one to remove the warning.