Merge bitcoin/bitcoin#28960: kernel: Remove dependency on CScheduler

d5228efb5391b31a9a0673019e43e7fa2cd4ac07 kernel: Remove dependency on CScheduler (TheCharlatan)
06069b3913dda048f5d640a662b0852f86346ace scripted-diff: Rename MainSignals to ValidationSignals (TheCharlatan)
0d6d2b650d1017691f48c9109a6cd020ab46aa73 scripted-diff: Rename SingleThreadedSchedulerClient to SerialTaskRunner (TheCharlatan)
4abde2c4e3fd9b66394b79874583bdc2a9132c36 [refactor] Make MainSignals RAII styled (TheCharlatan)
84f5c135b8118cbe15b8bfb4db80d61237987f64 refactor: De-globalize g_signals (TheCharlatan)
473dd4b97ae40e43e1a1a97fdbeb40be4855e9bc [refactor] Prepare for g_signals de-globalization (TheCharlatan)
3fba3d5deec6d7bae33823b8da7682f9b03d9deb [refactor] Make signals optional in mempool and chainman (TheCharlatan)

Pull request description:

  By defining a virtual interface class for the scheduler client, users of the kernel can now define their own event consuming infrastructure, without having to spawn threads or rely on the scheduler design.

  Removing `CScheduler` also allows removing the thread and exception modules from the kernel library.

  To make the `CMainSignals` class easier to use from a kernel library perspective, remove its global instantiation and adopt RAII practices.

  Renames `CMainSignals` to `ValidationSignals`, which more accurately describes its purpose and scope.

  Also make the `ValidationSignals` in the `ChainstateManager` and CTxMemPool` optional. This could be useful in the future for using or testing these classes without having to instantiate any form of signal handling.

  ---

  This PR is part of the [libbitcoinkernel project](https://github.com/bitcoin/bitcoin/issues/27587). It improves the kernel API and removes two modules from the kernel library.

ACKs for top commit:
  maflcko:
    re-ACK d5228efb5391b31a9a0673019e43e7fa2cd4ac07 🌄
  ryanofsky:
    Code review ACK d5228efb5391b31a9a0673019e43e7fa2cd4ac07. Just comment change since last review.
  vasild:
    ACK d5228efb5391b31a9a0673019e43e7fa2cd4ac07
  furszy:
    diff ACK d5228ef

Tree-SHA512: e93a5f10eb6182effb84bb981859a7ce750e466efd8171045d8d9e7fe46e4065631d9f6f533c5967c4d34c9bb7d7a67e9f4593bd4c5b30cd7b3bbad7be7b331b

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