Add a new TransportProtocol enum to the TransportLayer, rename the getEgressLayer function to getDefaultEgressLayer, and modify the getFirstTransportLayer<T> function to become getTransportLayer(TransportProtocol).

Also modify the createWithConfig function to start up an egress-only gRPC transport layer if the SPM-3902 feature flag is enabled. Ensure that the egress grpc layer is started regardless of whether TLS is enabled.

See the design for more details.