Enhance P4RT specification for ControllerPacketMetadata with regards to user defined types.

[jonathan-dilorenzo]

As per comments on https://github.com/p4lang/p4runtime/pull/422:

As a related point, It seems like the P4RT spec doesn't have any examples of a ControllerPacketMetadata with type_name being utilized. Do you think it'd be valuable to add such an example and show corresponding P4NewTypeSpec?

Originally posted by @chrispsommers in https://github.com/p4lang/p4runtime/pull/422#discussion_r1188836708