[addrman] Move CAddrMan::Serialize to cpp file

Summary:
Partial backport of core#22740:
https://github.com/bitcoin/bitcoin/pull/22740/commits/1622543cf42feb810d8ea9e7b3238d21f1427c17

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12283