Add support for by-ref-like (ref struct) parameter types such as Span<T> and ReadOnlySpan<T>

[stakx]

Unlike my earlier draft #664, this PR will not require DynamicProxy user code to set up any value converters for byref-like parameters.

Instead, any by-ref-like argument will automatically get substituted in IInvocation with a "reference" type:

• SpanArgument<T> SpanProxy<T> SpanReference<T> for Span<T> values
• ReadOnlySpanArgument<T> ReadOnlySpanProxy<T> ReadOnlySpanReference<T> for ReadOnlySpan<T> values
• ByRefLikeProxy ByRefLikeReference on .NET 8 for any non-span by-ref-like values
• ByRefLikeArgument<TByRefLike> ByRefLikeProxy<TByRefLike> ByRefLikeReference<TByRefLike> on .NET 9+ for any non-span by-ref-like values of type TByRefLike

Each of these (except the non-generic ByRefLikeReference) has a ref-returning Value property for accessing the actual value.

These class types are essentially references to the actual by-ref-like parameters. They use unmanaged pointers (void*) under the hood. I've added a big comment in the ByRefLikeReference.cs code file explaining why I think this is safe.

public interface IFoo
{
    void A(Span<char> characters);
    void B(ref Span<char> characters);
}

public void FooAInterceptor : IInterceptor
{
    public void Intercept(IInvocation invocation);
    {
        var charactersRef = invocation.Arguments[0] as SpanReference<char>;
        Span<char> characters = charactersRef.Value;
    }
}

public void FooBInterceptor : IInterceptor
{
    public void Intercept(IInvocation invocation);
    {
        var charactersRef = invocation.Arguments[0] as SpanReference<char>;
        Span<char> characters = charactersRef.Value;
        charactersRef.Value = characters[0..1]);
    }
}

I'm not quite done yet:

• add support for reading/writing by-ref-like argument values and propagating them through the interception pipeline
• add support for by-ref-like return values (as noted in a TODO in MethodWithInvocationGenerator.cs)
• update / replace unit tests in ByRefLikeTestCase (which still expect by-ref-like parameters to default / get nullified)
• write a documentation article on how to properly interact with by-ref-like arguments in IInvocation → preview it here
• update ref/ contract files
• update the changelog

Will fix #651 and close #663 once completed and merged.

[stakx]

This TODO needs to be implemented so we can return by-ref-like values, too.

[stakx]

Since we'll probably be adding a documentation page regarding by-ref-like values during interception, perhaps we could add exception messages with a link to the documentation page for more info...?

[stakx]

Should implement ConstructorInfo caching here, as the "todo" comment suggest.

Since Span<T> and ReadOnlySpan<T> are commonly encountered types in the FCL, perhaps constructed generic types deriving from these should also be in the cache.

[stakx]

An alternative to calling IInvocation.GetArgumentValue would be to query IInvocation.Arguments once, and then access the returned arguments array directly for every by-ref-like parameter? That might be more efficient for methods having more than one by-ref-like-typed parameter.

[stakx]

Implement this "todo" for by-ref-like-typed return parameter support. And don't forget to also Invalidate the wrapper object for the return type.