Add `#[rpc]` attribute to user-defined functions #2834

Job	Run time
clippy	1m 45s
license-guard	29s
doc-lints	47s
godot-itest (macos)	1m 22s
run-examples	1m 23s
godot-itest (windows)	3m 30s
rustfmt	10s
unit-test	2m 39s
godot-itest (linux-full)	2m 22s
godot-itest (linux-features-experimental)	2m 3s
godot-itest (linux-4.1)	1m 13s
godot-itest (linux-4.2)	1m 12s
godot-itest (linux-memcheck)	2m 19s
ci-status	1s
	21m 15s

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