Refactor 'client_info' support. (#7849)

* Add 'user_agent' and 'extra_headers' properties to 'Connection'.

   Deprecate the 'USER_AGENT' and '_EXTRA_HEADERS' class-level attributes.

* Add 'client_info' parameter to 'Connection' ctor.

* Implement 'Connection.user_agent' via its '_client_info'.

* Ensure 'X-Goog-API-Client' header is always passed.

* Create/use non-GAPIC-specific 'ClientInfo' class FBO HTTP/JSON clients.

  Derive the existing GAPIC class from it.
4 files changed