Add xUnit serializers for our types

[bash]

xUnit v3 has introduced a new way to support serialization of objects via an external IXunitSerializer (instead of implementing an interface on the object itself).
See: https://xunit.net/docs/getting-started/v3/custom-serialization

Implementing a serializer means that a type can be used in theory data (I think xUnit v3 is a lot more strict about this, not even showing the individual rows anymore if a type is not serializable).

This PR adds serializers for Option<T>, Either<L, R> and Unit.
There is no serializer for Result<T> since Exceptions are not xUnit-serialisable.
I also left out EitherOrBoth<L, R> for now (rarely used).

The serializers are discovered via assembly-level attributes implementing IRegisterXunitSerializerAttribute. Funcky.Xunit.v3.targets automatically adds these attributes to projects depending on Funcky.Xunit.v3 for a frictionless out-of-the-box experience :)

I also had to introduce two marker interfaces: IOption and IEither as IRegisterXunitSerializerAttribute does not support open generics (e.g. typeof(Option<>)).

Open Questions:

• Instead of having three attributes (one for each serializer). We could have one attribute that supports all types and a "meta"-serializer that calls the other serializers. Advantage: smaller API surface.

[Mafii]

I'm not 100% sure what upsides this adds, maybe you need to explain it to me.

curretly I don't think I want to accept it, as the interfaces add noise to the types...

[bash]

I'm not 100% sure what upsides this adds, maybe you need to explain it to me.

I feel like I explain the motivation fairly well in the PR description:

Implementing a serializer means that a type can be used in theory data (I think xUnit v3 is a lot more strict about this, not even showing the individual rows anymore if a type is not serializable).