Add `#[rpc]` attribute to user-defined functions #2840

Job	Run time
doc-lints	37s
license-guard	31s
clippy	1m 43s
godot-itest (macos)	56s
godot-itest (windows)	2m 53s
run-examples	1m 33s
rustfmt	18s
unit-test	2m 28s
godot-itest (linux-full)	2m 19s
godot-itest (linux-features-experimental)	2m 3s
godot-itest (linux-4.1)	1m 31s
godot-itest (linux-4.2)	1m 26s
godot-itest (linux-memcheck)	2m 9s
ci-status	1s
	20m 28s

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