[RFC][Generator] Add support for indexers

Apple usually provides classes that are collection of objects that provide a GetObject (obj) and SetObject(obj, index) methods. On C# we will like to expose these methods AND and indexer so that the users can do obj[i].

The binding could use a very similar syntax as that one used for properties with a Wrap:

```csharp
MTLBlitPassSampleBufferAttachmentDescriptor this[nuint i] {
[Wrap ("GetObject (i)")]
get;
[Wrap ("SetObject (value, i);")]
set;
}
```

The generator can later write:
```csharp
MTLBlitPassSampleBufferAttachmentDescriptor this[nuint i] {
get => GetObject (i);
set => SetObject (value, i);
}
```

This will safe us a good amount of manual code for this classes.