Make p2p interface specifications executable/testable

For each fork's version of the [p2p-interface specifications](https://github.com/ethereum/consensus-specs/blob/master/specs/phase0/p2p-interface.md), we should make the verification checks executable functions. Eg:

https://github.com/ethereum/consensus-specs/blob/9c68080a5b3fe898ddd633488152449f53a0a624/specs/phase0/p2p-interface.md?plain=1#L437-L464

Then, we can create new test formats & reference tests for clients.

This is not an urgent feature, but it would be very nice to have. I would like to do this myself after the Fulu upgrade.