Implement dynTrait for Gd<T: TraitA> #2961

Job	Run time
clippy	1m 45s
doc-lints	45s
godot-itest (macos)	59s
license-guard	28s
godot-itest (windows)	3m 4s
run-examples	1m 11s
rustfmt	34s
godot-itest (linux-full)	2m 32s
unit-test	2m 13s
godot-itest (linux-features-experimental)	1m 58s
godot-itest (linux-4.1)	1m 14s
godot-itest (linux-4.2)	1m 4s
godot-itest (linux-memcheck)	2m 33s
ci-status	2s
	20m 22s

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