net_processing: move Peer definition to .cpp

Summary:
Completes backport of core#20811:
https://github.com/bitcoin/bitcoin/pull/20811/commits/c97f70c861ac6959b8116a9bca3031edeb2b2aaa

Depends on D10890.

Ref T1696.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1696

Differential Revision: https://reviews.bitcoinabc.org/D10892