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

This reverts commit 3d04e025.

Now that gRPC_INSTALL is forced off if using bundled third-party
libraries, it no longer makes sense to set it OFF by default for MSVC.

However, we do want to set it OFF by default if gRPC is being built as a
subproject of another project.

If gRPC is not getting its (link-time) dependencies from the system, it
should not be attempting installation.

This reverts commit c019e057.

The next release of c-ares will install a CMake package (when built
using CMake), but it will be called "c-ares", not "CARES".