[addrman] Introduce CAddrMan::Impl to encapsulate addrman implementation.
Summary:
Introduce the pimpl pattern for CAddrMan to separate the implementation details from the externally used object representation. This reduces compile-time dependencies and conceptually clarifies AddrMan's interface from the implementation specifics. Since the unit & fuzz tests currently rely on accessing CAddrMan internals, this commit introduces addrman_impl.h, which is exclusively imported by addrman.cpp and test files.
Partial backport of core#22950:
https://github.com/bitcoin/bitcoin/pull/22950/commits/8af5b54f973e11c847345418d8631bc301b96130
Depends on D12328.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D12329