Implement dynTrait for Gd<T: TraitA> #2971

Job	Run time
doc-lints	57s
clippy	1m 48s
godot-itest (macos)	1m 21s
license-guard	34s
godot-itest (windows)	3m 6s
run-examples	1m 39s
rustfmt	17s
godot-itest (linux-full)	2m 18s
unit-test	2m 13s
godot-itest (linux-features-experimental)	2m 17s
godot-itest (linux-4.1)	1m 26s
godot-itest (linux-4.2)	1m 41s
godot-itest (linux-memcheck)	2m 21s
ci-status	2s
	22m 0s

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