Add `#[rpc]` attribute to user-defined functions #2833

Job	Run time
clippy	1m 53s
godot-itest (macos)	1m 31s
run-examples	1m 20s
doc-lints	51s
godot-itest (windows)	2m 48s
rustfmt	26s
godot-itest (linux-full)	2m 54s
unit-test	2m 26s
license-guard	35s
godot-itest (linux-features-experimental)	1m 55s
godot-itest (linux-4.1)	1m 8s
godot-itest (linux-4.2)	1m 10s
godot-itest (linux-memcheck)	2m 7s
ci-status	3s
	21m 7s

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