Alternative to defining a bunch of MP-specific frames

[martinthomson]

With the new explicit path identifier, maybe I can suggest an improvement.

Look at Victor's https://datatracker.ietf.org/doc/draft-vvv-quic-namespaces/ It strikes me that the trick he is using with the NS frame there might be useful for MP QUIC. Only path-specific frames would be affected. You can even start with a default value of the current path.