[ffigen] Objective-C interface static methods and `ffiNative:`

When using

```yaml
ffiNative:
```

With toplevel methods in Objective-C we directly invoke via an `@Native`

```dart
@Native()
external void play();
```

But if I change it to a static method on an interface we get a reflective implementation, that does dynamic symbol lookup.

```dart
class MyMediaPlayer extends objc.NSObject {
static void play() {
_objc_msgSend_2(_class_MyMediaPlayer, _sel_play);
}
```

@liamappelbe Is it needed that we do something reflective here? Or can we use a `@Native` here as well? If we'd want to do that, we'd have to deal with the name mangling?

```
$ nm test/native_objc_test/static_func_test.dylib
0000000000003d08 t +[StaticFuncTestObj newWithCounter:] // <-- name mangled?
0000000000003de4 t -[StaticFuncTestObj dealloc]
0000000000003d44 t -[StaticFuncTestObj initWithCounter:]
0000000000003dac t -[StaticFuncTestObj setCounter:]
U _OBJC_CLASS_$_NSObject
00000000000080f8 S _OBJC_CLASS_$_StaticFuncTestObj
00000000000080f0 S _OBJC_IVAR_$_StaticFuncTestObj.counter
U _OBJC_METACLASS_$_NSObject
0000000000008120 S _OBJC_METACLASS_$_StaticFuncTestObj
0000000000003ec8 s __OBJC_$_CLASS_METHODS_StaticFuncTestObj
0000000000003ee0 s __OBJC_$_INSTANCE_METHODS_StaticFuncTestObj
0000000000008048 s __OBJC_$_INSTANCE_VARIABLES_StaticFuncTestObj
0000000000008070 s __OBJC_CLASS_RO_$_StaticFuncTestObj
0000000000008000 s __OBJC_METACLASS_RO_$_StaticFuncTestObj
U __objc_empty_cache
0000000000003b30 T _getBlockRetainCount
0000000000003b54 T _getObjectRetainCount
0000000000003ba4 T _isReadableMemory
U _mach_task_self_
U _mach_vm_region
U _objc_alloc
U _objc_msgSend
0000000000003e88 s _objc_msgSend$initWithCounter:
0000000000003ea8 s _objc_msgSend$newWithCounter:
U _objc_msgSendSuper2
U _objc_retain
0000000000003ca4 T _staticFuncOfBlock
0000000000003c90 T _staticFuncOfNullableObject
0000000000003c7c T _staticFuncOfObject
0000000000003cb8 T _staticFuncReturnsRetained
0000000000003ce4 T _staticFuncReturnsRetainedArg
```

Or are there other reasons why we can't do this?

This would be mostly for performance reasons. For tree-shaking, we would generate different annotations anyway (https://github.com/dart-lang/native/issues/1099).