Generic generators

[AlexeyRaga]

This PR adds the ability to register custom generators for generic type.

It is the main reason FSCheck is used alongside with Hedgehog as a "fallback" solution in my company: we use a lot of generic data structures, mostly written in C#.
Having the ability to register generators for custom generic types simplified writing tests a lot.

To register a generator for a generic type we can now:

FSharp:

// Create a type in which generators are defined:
type GenericTestGenerators =

    // Create a static member that returns a generator for a generic structure
    // It can take a number of Gen<_> parameters if it needs to
    static member MyEitherGen<'a, 'b>(genLeft: Gen<'a>, genRight: Gen<'b>) : Gen<Either<'a, 'b>> = ???

    static member MyMaybeGen<'a>(genA: Gen<'a>): Gen<Maybe<'a>> = ???

CSharp:

public sealed class GenericTestGenerators
{
    public static Gen<Either<TLeft, TRight>> MyEitherGen<TLeft, TRight>(Gen<TLeft> genA, Gen<TRight> genB) => ???;
    public static Gen<Maybe<T>> MyMaybeGen<T>(Gen<T> gen) => ???;
}

And then these generators can be registered using addGenerators<'a> function:

FSharp:

let config = GenX.defaults |> AutoGenConfig.addGenerators<GenericTestGenerators>

CSharp:

var config = GenX.defaults.WithGenerators<GenericTestGenerators>();

Ref: #77

[TysonMN]

Why did you change the .NET version?

[AlexeyRaga]

@TysonMN
Because .NET 6 reached Out Of Support last year (in November, if I remember correctly).
.NET 8 is the next LTS which is supported until the end of 2026.

[TysonMN]

So it wasn't needed to add this feature then?

If not, then please make a separate PR with a single commit that upgrades .NET. In the commit message body, put the explanation that you have given here.

[AlexeyRaga]

I can check, but I believe that it won't compile under .NET 6.
I can make a PR so we can upgrade to .NET 8 first, and then rebase this one on top.

[AlexeyRaga]

I mean, .NET 8 is needed for the feature the way it is now.
I just decided not to take any extra effort to add .NET 6 workarounds and simply use the officially supported version.

This way we can say that it is needed to add the feature.

[TysonMN]

I can make a PR so we can upgrade to .NET 8 first, and then rebase this one on top.

Yes. Two PRs in that order would be good.

[AlexeyRaga]

@TysonMN
OK, let's merge #79 and then I'll rebase this one on top

[AlexeyRaga]

@TysonMN As discussed, I have rebased this branch on top of .NET 8 enabled master.
Please check again.