net: CNetAddr: add support to (un)serialize as ADDRv2

Summary:
Partial backport (3/4) of core PR19845:
https://github.com/bitcoin/bitcoin/pull/19845/commits/e0d73573a37bf4b519f6f61e5678572d48a64517

Depends on D9186 and D9187.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9188