Mono support for new function pointer APIs

[jgh07]

This acts as a tracking issue for the Mono implementation of the new APIs proposed in #75348. The following APIs will have to be implemented on the corresponding Mono-specific subtypes:

 namespace System
 {
    public abstract partial class Type
    {
        public virtual Type MakeFunctionPointerType(Type[]? parameterTypes, bool isUnmanaged = false) => throw new NotSupportedException(SR.NotSupported_SubclassOverride);
    }
 }

 namespace System.Reflection.Emit
 {
    public abstract partial class ILGenerator
    {
        public virtual void EmitCalli(Type functionPointerType) => throw new NotImplementedException();
    }
 }