Add StronglyTypedId<T> generic attribute for C# 11+

[kzu]

Now that C# (11+) supports generic attributes, it would be nice to be able to annotate a type with the generic version to specify the backing type:

[StronglyTypedId<int>]
public partial struct UserId { }

[andrewlock]

Thanks for the suggestion, but unfortunately this won't work with the new design. The backing type isn't an unconstrained type, it's one of a specific enum values, or a string. This was discussed further in #102.

[kzu]

Hm, it's kinda losing its original simplicity-focused approach.