Add support for using ssl_credentials and attempt_direct_path in google.api_core.grpc_helpers.create_channel

[parthea]

Related to PR #583, it is not supported to use attempt_direct_path and ssl_credentials because we use grpc.compute_engine_channel_credentials for Direct Path which doesn't support passing ssl_credentials. Support for mTLS and Direct Path will be added later.

See the comment below

    # if `ssl_credentials` is set, use `grpc.composite_channel_credentials` instead of
    # `grpc.compute_engine_channel_credentials` as the former supports passing
    # `ssl_credentials` via `channel_credentials` which is needed for mTLS.