Remove a follow-on internal that was added post-internalization, keep the templated QPS worker rather than going back to internal API

Test credentials are passed with channel arg. Renamed macros and refactored code as per suggestions. Renamed test to proxy_auth and changed it to use simple_request instead of a payload.

Adding connect auth feature. Proxy-Authorization header is being inserted when user creds are present in uri

require using grpc_slice

Disbable a specific check on async_end2end_test and revisit later if that expectation was really required by the API

- In C++, we need a constant for the max lifetime.
- In C, make sure that we crop the lifetime in the credentials object
itself and not just later during the creation of the token. This will
allow the refresh to occur based on the actual token lifetime as opposed
to the one from the user (which may be cropped).

This target is used internally.

Enable internal credential types.