From previous PRs, we've already added support for include_type_name to
the get mapping API. We had also taken an approach to the HLRC where the
server-side `GetMappingResponse#fromXContent` could only handle typeless
input.
This PR updates the HLRC for 'get mapping' to be in line with our new approach:
* Add a typeless 'get mappings' method to the Java HLRC, that accepts new
client-side request and response objects. This new response only handles
typeless mapping definitions.
* Switch the old version of `GetMappingResponse` back to expecting typed
mappings, and deprecate the corresponding method on the HLRC.
Finally, the PR also does some small, related clean-up around 'get field mappings'.
b1735aa93b