Use string Literal for str-based matching

[Zeitsperre]

I think we could simply map it to xclim's STRING ? Unless there's a simple way to detect if the allowed values' type, my guess is that we don't really need Literals for anything else than strings. So here :

xclim/xclim/core/utils.py

Lines 659 to 660 in 98c251c

	if annot.issuperset({"str"}):
	return InputKind.STRING

And if we do that, maybe we could apply the same for all op and method arguments throughout the library ? And maybe use the annotation to parse the choices property of xclim.core.indicator.Parameter ?

Originally posted by @aulemahal in #1796 (comment)