Generalize trampoline code to allow reuse and reduce code size

### Steps to Reproduce

1. Build xamarin-macios
2. Open generated file `Trampolines.g.cs`

### Expected Behavior

A single instance of trampolines to invoke a `NSObject` / `INative`

### Actual Behavior

Each case is specialized to it's own type, e.g.

```csharp
[Preserve (Conditional=true)]
[BindingImpl (BindingImplOptions.GeneratedCode | BindingImplOptions.Optimizable)]
unsafe void Invoke (global::AVFoundation.AVAsynchronousCIImageFilteringRequest obj)
{
invoker (blockPtr, obj == null ? IntPtr.Zero : obj.Handle);
}
```

Which lead to code duplication, the linker will remove unused ones, but it still won't go down to 1 (and the linker can be disable, e.g. interpreter).

E.g. There are 171 instances (for iOS on xcode11 branch) of
```csharp
invoker (blockPtr, obj == null ? IntPtr.Zero : obj.Handle);
```

That affects app size and build time (more to AOT).

There are more candidates for optimization - but that one seems easy and quite frequent.