Add `#[rpc]` attribute to user-defined functions #2832

Job	Run time
license-guard	29s
clippy	1m 48s
doc-lints	46s
godot-itest (macos)	45s
run-examples	26s
rustfmt	23s
godot-itest (windows)	2m 4s
unit-test	2m 41s
godot-itest (linux-full)	2m 6s
godot-itest (linux-features-experimental)	2m 6s
godot-itest (linux-4.1)	42s
godot-itest (linux-4.2)	47s
godot-itest (linux-memcheck)	1m 2s
ci-status	2s
	16m 7s

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