fix: rewrite plugin background module to not use broken ABI

Summary

• fix: rewrite plugin background module to not use broken ABI

Closes #1072 (closed) Closes #1191 (closed)

• refactor: simplify internal plugin event handling code

Remove the redundant indirection via the handle_event_sync function

• feat: increase on_shutdown trigger timeout to 30 seconds

• refactor: move plugin metrics storage into plugin_manager

• chore: fix the annoying static mut compilation warnings

• refactor: introduce ability to initialize a dummy Node for testing

• refactor: use non-joinable fibers for internal loops

• feat!: ServiceId now stores SmolStr instead of String

This should eliminate a number of redundant memory allocations. Although this improvement is probably indistinguishably tiny compared to all other redundant allocations we're doing...

• fix: remove UB from plugin background shutdown timeout configuration

Part of #1191 (closed)

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• Cherry-pick to: none
• Docs follow-up: not necessary
• Changelog is updated