net_processing: move AddToCompactExtraTransactions into PeerManagerImpl

Summary:
Allows making vExtraTxnForCompact and vExtraTxnForCompactIt member vars
instead of globals.

This is a backport of core#21148 [13/14]
https://github.com/bitcoin/bitcoin/pull/21148/commits/eeeafb324ef6057f40b5c5fdd8464110e809b0f7

Depends on D11491

Test Plan:
With clang and debug:
ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11492