Support numeric constraints for Decimal values

[corentin-regent]

Description

I am currently using msgspec for parsing financial (cryptocurrency) data from various exchanges. Precision in this context is crucial, so I use Decimal values quite a lot. Though for safety and documentation, I would also like to set constraints for these values, however msgspec currently does not support it.

Supporting numeric constraints for the Decimal type would add consistency with the int and float types:

>>> msgspec.json.decode('123.456', type=Annotated[float, Meta(gt=0)])
123.456
>>> msgspec.json.decode('123.456', type=Annotated[Decimal, Meta(gt=0)])
TypeError: Can only set `gt` on a numeric type - type `typing.Annotated[decimal.Decimal, msgspec.Meta(gt=0)]` is invalid

Additionally (may be a separate feature), supporting the multiple_of constraint with a Decimal argument (e.g. Meta(multiple_of=Decimal('0.1'))) would provide users with a workaround to the false negatives due to precision loss, from this issue.