[addrman] Move CAddrMan::Unserialize to cpp file

Summary:
Partial backport of core#22740:
https://github.com/bitcoin/bitcoin/pull/22740/commits/a65053f1d44c72c43754a5d5aeb684fc1fca0300

Depends on D12283.

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12284