Implement dynTrait for Gd<T: TraitA> #2905

Job	Run time
clippy	1m 59s
doc-lints	52s
godot-itest (macos)	1m 5s
license-guard	31s
godot-itest (windows)	3m 29s
run-examples	1m 26s
rustfmt	19s
godot-itest (linux-full)	2m 52s
unit-test	2m 25s
godot-itest (linux-features-experimental)	2m 20s
godot-itest (linux-4.1)	1m 15s
godot-itest (linux-4.2)	1m 12s
godot-itest (linux-memcheck)	2m 34s
ci-status	2s
	22m 21s

Provide feedback