Simpler bindings for [StrongDictionary]

We can simplify them by re-use the smart enums pattern, inlining the `[Field]` into the dictionary. E.g.

Today
```csharp
[Watch (6,0), TV (13,0), Mac (10,15), iOS (13,0)]
[Internal]
[Static]
interface MLFeatureValueImageOptionKeys {

[Field ("MLFeatureValueImageOptionCropRect")]
NSString CropRectKey { get; }

[Field ("MLFeatureValueImageOptionCropAndScale")]
NSString CropAndScaleKey { get; }
}

[Watch (6,0), TV (13,0), Mac (10,15), iOS (13,0)]
[StrongDictionary ("MLFeatureValueImageOptionKeys")]
interface MLFeatureValueImageOption {
CGRect CropRect { get; set; }
VNImageCropAndScaleOption CropAndScale { get; set; }
}
```

Future
```csharp
[Watch (6,0), TV (13,0), Mac (10,15), iOS (13,0)]
[StrongDictionary]
interface MLFeatureValueImageOption {
[Field ("MLFeatureValueImageOptionCropRect")]
CGRect CropRect { get; set; }
[Field ("MLFeatureValueImageOptionCropAndScale")]
VNImageCropAndScaleOption CropAndScale { get; set; }
}
```

Pros:
* Easier to review code (no scrolling to compare)
* Availability attributes could not mismatch (intro only checks `[Field]`)

Cons:
* generator change :)