Encapsulate warnings in generalized node::Warnings and remove globals

[stickies-v]

This PR:

• moves warnings from common to the node library and into the node namespace (as suggested in rpc: return warnings as an array instead of just a single one #29845 (comment))
• 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.

[DrahtBot]

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Concept ACK	TheCharlatan

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #29432 (Stratum v2 Template Provider (take 3) by Sjors)
• #29415 (Broadcast own transactions only via short-lived Tor or I2P connections by vasild)
• #29039 (versionbits refactoring by ajtowns)
• #28830 ([refactor] Check CTxMemPool options in ctor by TheCharlatan)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

[DrahtBot]

🚧 At least one of the CI tasks failed. Make sure to run all tests locally, according to the
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Possibly this is due to a silent merge conflict (the changes in this pull request being
incompatible with the current code in the target branch). If so, make sure to rebase on the latest
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_{Debug: https://github.com/bitcoin/bitcoin/runs/24704508399}

[TheCharlatan]

Concept ACK on removing the warnings globals.

[stickies-v]

Force-pushed to address compilation failure on non-macOS systems:

• moved GetNodeWarnings() (in rpc/util.cpp) to node::GetWarningsForRpc() (in node/warnings.cpp). Since rpc/util.cpp is in common, this causes issues after warnings are moved to node. I don't love this approach, but it seemed like the least bad one - open to suggestions if anyone has any?
• updated bitcoin-chainstate.cpp to the new kernel::Notifications interface