net: add V2Transport class with subset of BIP324 functionality

This introduces a V2Transport with a basic subset of BIP324 functionality:
* no ability to send garbage (but receiving is supported)
* no ability to send decoy packets (but receiving them is supported)
* no support for short message id encoding (neither encoding or decoding)
* no waiting until 12 non-V1 bytes have been received
* (and thus) no detection of V1 connections on the responder side
  (on the sender side, detecting V1 is not supported either, but that needs
  to be dealt with at a higher layer, by reconnecting)

Author: sipa