simplify GossipVerifier types using APeerManager

[johncantrell97]

This simplifies the types but also allows an end-user (RGS in this case) to use a PeerManager that does not use P2PGossipSync as their RoutingMessageHandler.

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (37150b4) 88.63% compared to head (6765967) 88.62%.

Files	Patch %	Lines
lightning-block-sync/src/gossip.rs	0.00%	3 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2773      +/-   ##
==========================================
- Coverage   88.63%   88.62%   -0.01%     
==========================================
  Files         115      115              
  Lines       90446    90446              
  Branches    90446    90446              
==========================================
- Hits        80170    80161       -9     
- Misses       7875     7879       +4     
- Partials     2401     2406       +5     

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[TheBlueMatt]

Nice