Merge bitcoin/bitcoin#30058: Encapsulate warnings in generalized node::Warnings and remove globals

260f8da71a refactor: remove warnings globals (stickies-v)
9c4b0b7ce4 node: update uiInterface whenever warnings updated (stickies-v)
b071ad9770 introduce and use the generalized `node::Warnings` interface (stickies-v)
20e616f864 move-only: move warnings from common to node (stickies-v)
bed29c481a refactor: remove unnecessary AppendWarning helper function (stickies-v)

Pull request description:

  This PR:
  - moves warnings from common to the node library and into the node namespace (as suggested in https://github.com/bitcoin/bitcoin/pull/29845#discussion_r1570069541)
  - generalizes the warnings interface to `Warnings::Set()` and `Warnings::Unset()` methods, instead of having a separate function and globals for each warning. As a result, this simplifies the `kernel::Notifications` interface.
  - removes warnings.cpp from the kernel library
  - removes warning globals
  - adds testing for the warning logic

  Behaviour change introduced:
  - the `-alertnotify` command is executed for all `KernelNotifications::warningSet` calls, which now also covers the `LARGE_WORK_INVALID_CHAIN` warning
  - the GUI is updated automatically whenever a warning is (un)set, covering some code paths where it previously wouldn't be, e.g. when `node::AbortNode()` is called, or for the `LARGE_WORK_INVALID_CHAIN` warning

  Some discussion points:
  - ~is `const std::string& id` the best way to refer to warnings? Enums are an obvious alternative, but since we need to define warnings across libraries, strings seem like a straightforward solution.~ _edit: updated approach to use `node::Warning` and `kernel::Warning` enums._

ACKs for top commit:
  achow101:
    ACK 260f8da71a
  ryanofsky:
    Code review ACK 260f8da71a. Only change since last review was rebasing
  TheCharlatan:
    Re-ACK 260f8da71a

Tree-SHA512: a3fcedaee0d3ad64e9c111aeb30665162f98e0e72acd6a70b76ff2ddf4f0a34da4f97ce353c322a1668ca6ee4d8a81cc6e6d170c5bbeb7a43cffdaf66646b588

doc/release-notes-30058.md

@@ -0,0 +1,7 @@
+- When running with -alertnotify, an alert can now be raised multiple
+times instead of just once. Previously, it was only raised when unknown
+new consensus rules were activated, whereas the scope has now been
+increased to include all kernel warnings. Specifically, alerts will now
+also be raised when an invalid chain with a large amount of work has
+been detected. Additional warnings may be added in the future.
+(#30058)