Implement dynTrait for Gd<T: TraitA> #2906

Job	Run time
license-guard	30s
doc-lints	51s
godot-itest (macos)	56s
clippy	1m 50s
godot-itest (windows)	1m 59s
run-examples	1m 20s
godot-itest (linux-full)	2m 40s
rustfmt	30s
unit-test	2m 24s
godot-itest (linux-features-experimental)	2m 21s
godot-itest (linux-4.1)	1m 7s
godot-itest (linux-4.2)	1m 31s
godot-itest (linux-memcheck)	2m 38s
ci-status	1s
	20m 38s

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