feat(grpc): Plumb credentials in the channel

[arjan-bal]

This PR has the following changes:

• Have channel::new accept an Arc<C: ChannelCredentials> to enable using the same credentials, after type erasure, for out-of-band channels.
• Have ChanneCredsInternal::connect accept references to some of its params to avoid cloning Arced values when possible.
• Plumb credentials through the channel to the transport and have the tonic transport perform the security handshake.
• Plumb call credentials through the channel and have the subchannel ensure correct security levels and add metadata.
• Add a test for a TLS enabled gRPC client communicating with a TLS enabled tonic server.
• In channel.rs, use the URL path to determine the authority instead of the URL authority. The URL authority is only meant to be used by name resolvers.