p2p: add CInv block message helper methods

Summary:
Partial backport of core#19610:
https://github.com/bitcoin/bitcoin/pull/19610/commits/471714e1f024fb3b4892a7a8b34a76b83a13fa19

Depends on D9500.

Test Plan:

ninja all check
ninja bitcoin-fuzzers

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9501