(feat): add setting to retain categories

[ilan-gold]

• Closes Option to retain categories in columns when subsetting anndata object.  #890
• Tests added
• Release note added (or unnecessary)

[flying-sheep]

Hmm, maybe this can be simplified to

Suggested change

	allowed_values: Sequence[str] | None = None,
	cast: Callable[[Any], T] = lambda x: x,
	cast: Callable[[Any], T] | type[Enum] = lambda x: x,

• if it’s a callable, just call it and let the error go through if it fails
• if it’s an enum type, use cast[val] to get it by name

[flying-sheep]

I typed cast[val] for a reason.

class Test(Enum):
    a = 1
    b = 2

assert Test["a"] is Test.a
assert Test(1) is Test.a

[ilan-gold]

Forgot to add second change, apologies.

[flying-sheep]

See #1340 (comment)