Implement dynTrait for Gd<T: TraitA> #2925

Job	Run time
clippy	1m 49s
license-guard	30s
godot-itest (macos)	1m 30s
doc-lints	51s
godot-itest (windows)	2m 48s
run-examples	1m 19s
rustfmt	14s
godot-itest (linux-full)	2m 31s
unit-test	2m 32s
godot-itest (linux-features-experimental)	2m 6s
godot-itest (linux-4.1)	1m 10s
godot-itest (linux-4.2)	1m 5s
godot-itest (linux-memcheck)	2m 37s
ci-status	3s
	21m 5s

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