Implement dynTrait for Gd<T: TraitA> #2922

Job	Run time
rustfmt	14s
doc-lints	43s
clippy	1m 52s
godot-itest (macos)	56s
godot-itest (windows)	2m 13s
unit-test	2m 23s
godot-itest (linux-full)	2m 46s
godot-itest (linux-features-experimental)	2m 12s
license-guard	31s
godot-itest (linux-4.1)	1m 11s
godot-itest (linux-4.2)	1m 9s
run-examples	1m 23s
godot-itest (linux-memcheck)	2m 32s
ci-status	2s
	20m 7s

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