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merge-script 2b541eeb36 Merge bitcoin/bitcoin#34495 : Replace boost signals with minimal compatible implementation

242b0ebb5c btcsignals: use a single shared_ptr for liveness and callback (Cory Fields)
b12f43a0a8 signals: remove boost::signals2 from depends and vcpkg (Cory Fields)
a4b1607983 signals: remove boost::signals2 mentions in linters and docs (Cory Fields)
375397ebd9 signals: remove boost includes where possible (Cory Fields)
091736a153 signals: re-add forward-declares to interface headers (Cory Fields)
9958f4fe49 Revert "signals: Temporarily add boost headers to bitcoind and bitcoin-node builds" (Cory Fields)
34eabd77a2 signals: remove boost compatibility guards (Cory Fields)
e60a0b9a22 signals: Add a simplified boost-compatible implementation (Cory Fields)
63c68e2a3f signals: add signals tests (Cory Fields)
edc2978058 signals: use an alias for the boost::signals2 namespace (Cory Fields)
9ade3929aa signals: remove forward-declare for signals (Cory Fields)
037e58b57b signals: use forwarding header for boost signals (Cory Fields)
2150153f37 signals: Temporarily add boost headers to bitcoind and bitcoin-node builds (Cory Fields)
fd5e9d9904 signals: Use a lambda to avoid connecting a signal to another signal (Cory Fields)

Pull request description:

  This drops our dependency on `boost::signals2`, leaving `boost::multi_index` as the only remaining boost dependency for bitcoind.

  `boost::signals2` is a complex beast, but we only use a small portion of it. Namely: it's a way for multiple subscribers to connect to the same event, and the ability to later disconnect individual subscribers from that event.

  `btcsignals` adheres to the subset of the `boost::signals2` API that we currently use, and thus is a drop-in replacement. Rather than implementing a complex `slot` tracking class that we never used anyway (and which was much more useful in the days before std::function existed), callbacks are simply wrapped directly in `std::function`s.

  The new tests work with either `boost::signals2` or the new `btcsignals` implementation. Reviewers can verify
  functional equivalency by running the tests in the commit that introduces them against `boost::signals2`, then again with `btcsignals`.

  The majority of the commits in this PR are preparation and cleanup. Once `boost::signals2` is no longer needed, it is removed from depends. Additionally, a few CMake targets no longer need boost includes as they were previously only required for signals.

  I think this is actually pretty straightforward to review. I kept things simple, including keeping types unmovable/uncopyable where possible rather than trying to define those semantics. In doing so, the new implementation has even fewer type requirements than boost, which I believe is due to a boost bug. I've opened a PR upstream for that to attempt to maintain parity between the implementations.

  See individual commits for more details.

  Closes #26442.

ACKs for top commit:
  fjahr:
    Code review ACK 242b0ebb5c
  maflcko:
    re-review ACK 242b0ebb5c 🎯
  w0xlt:
    reACK 242b0ebb5c

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