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

Added all previous minor releases of gRPC to the matrix.  Modified the testsuite name format for better display test results in sponge.

Added c++ to interop_matrix.  Per earlier conversation, interop_matrix will not run against different compiler/toolchains so we are not addding _<runtime> to c++. This simplifies our setup and reduces unnecesary complexities in the infrastructure.

This has the disadvantage that we can only use the current tree to
collect evidence of overlap -- this means that it's possible for file
addition or deletion to force a CODEOWNERS update.

HOWEVER, it has the advantage that it's correct and propagates ownership
in the way that we want (alleviating murgatroid@ from having to approve
every additional file in the repository)

-) Paths should all be absolute to /
-) Computing the intersection of two globs should happen with the
full path in there, not just the directory's glob.