This allows `grpc_cli` to act with the specific client identity when using SSL. It does _not_ however set the cert when using Google Default Credentials, as it does not have the necessary options to provide a client cert and key.
This replaces several mutually-exclusive flags for a single selector flag which defines
the base channel type. It also allows child classes to selectively override the defaults
and provide additional channel types (e.g., LOAS in the mono repo).
Fixes issue #16060.
This allows the client to specify the host name of the remote server for the purposes of TLS certificate validation, useful for test certificates and machines with ephemeral IP
addresses with no associated DNS entries that have been assigned TLS certificates.
