net_processing: split PeerManager into interface and implementation classes

Summary:
Partial backport of core#20811:
https://github.com/bitcoin/bitcoin/pull/20811/commits/a568b82febb3ecbd5ebb7c3f9da27e762b0c68f6

Note that the cpp file deserves a good reordering, but that's out of scope for this diff.

Depends on D10886.

Ref T1696.

Test Plan:

ninja all check-all
ninja bitcoin-fuzzers

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1696

Differential Revision: https://reviews.bitcoinabc.org/D10887