[vm/ffi] ObjC: Make it possible to subclass/implement ObjC classes/protocols

There's use cases where it would be very convenient to make subclasses of ObjC classes (e.g. see [sub-classing NSOperationQueue](https://dart-review.googlesource.com/c/sdk/+/242062/1/runtime/lib/ffi_queue.mm)) and doing so in Dart.

Having this ability would avoid the need to write native code (and thereby not require changes to `dart:ffi` or adding flutter-specific plugins) - it could be done purely in Dart .

The most convenient way would be have Dart syntax for making subclasses of ObjC, possibly with some language restrictions and kernel lowering - similar to how the pure C-based FFI support is done.

Though a simpler way to get started would be providing utilities to procedurally lookup classes, create classes, add methods, ... (see [objc_allocateClassPair](https://developer.apple.com/documentation/objectivec/1418559-objc_allocateclasspair), [objc_addmethod](https://developer.apple.com/documentation/objectivec/1418901-class_addmethod))

/cc @brianquinlan @mraleph @dcharkes