Fix building proxies for classess with static interface members

[hazzik]

C# 8 (.NET Core 3.1) has introduced static interface members. Currently they are throwing "IndexOutOfRangeException: Index was outside the bounds of the array."
C# 11 (.NET 7) has introduced abstract/virtual interface members. Currently they are throwing "TypeLoadException: Signature of the body and declaration in a method implementation do not match."

In general static interface members should not be proxiable (same as regular static methods) with one exception of interface-only entities for which, if such interface would have an abstract static member it must be implemented by a proxy. However, I could not find a valid use case for this. In this case proxy will fail with "System.TypeLoadException: Virtual static method 'XXX' is not implemented on type 'YYYProxy' from assembly 'YYYProxyAssembly'"

Fixes #3295

[hazzik]

@bahusoid @fredericDelaporte, I want to know your opinion:

1. Shall I target 5.4.x or master?
2. Shall I upgrade test projects to .NET 7?

[bahusoid]

Shall I target 5.4.x or master?

5.4

Shall I upgrade test projects to .NET 7?

No opinion. It seems static members are tested without it.

with one exception of interface-only entities for which, if such interface would have an abstract static member it must be implemented by a proxy.

I don't get it. Can you provide an example?

[hazzik]

No opinion. It seems static members are tested without it.

I wrote my comment before I discovered that static interface members do not work in lower .NET (Core) versions.

I don't get it. Can you provide an example?

This is the modification of the test that will fail in .NET 7:

[Test(Description = "GH3295")]
public void VerifyProxyForInterfaceWithStaticAbstractMethod()
{
	var factory = new StaticProxyFactory();
	factory.PostInstantiate(
		typeof(IWithStaticMethods).FullName,
		typeof(object),
		new HashSet<System.Type> { typeof(INHibernateProxy), typeof(IWithStaticMethods) },
		null, null, null, true);

	var proxy = factory.GetProxy(1, null);
	Assert.That(proxy, Is.Not.Null);
}

There are two scenarios where this can be used:

1. Using <class name="Smth" proxy="{interface}"> option in the mapping (chapter 21.1.3 of the documentation):

   <class name="Person" proxy="IPerson">
     <id name="Id" />
     <property name="Name" />
   </class>

   public interface IPerson
   {
     int Id { get; }
     string Name { get; set; }
     static abstract void HelloWorld(); // <-- this will fail in .NET 7
   }
   
   public class Person : IPerson
   {
     public int Id { get; private set; }
     public string Name { get; set; }
     public static void HelloWorld() { }
   }

2. Another one is interface-only entities (<class name="{interface}">, vaguely described in 9.1.1. Table per class hierarchy and 9.1.2. Table per subclass of the documentation):

   <class name="IPerson">
     <id name="Id" />
     <property name="Name" />
   </class>

   public interface IPerson
   {
     int Id { get; }
     string Name { get; set; }
     static abstract void HelloWorld(); // <-- this will fail in .NET 7
   }

However, as I said, it does not make much sense for to.

[fredericDelaporte]

To enable the test for this case, yes we should switch net6.0 target to 7.0 in the test project.

Ok for the target branch.