NSAttributedString/NSMutableAttributedString missing substring constructors

Currently, `NSString` APIs allow only to pass a managed string.

This forces APIs which would typically just use a substring be forced to allocate a string for the substring itself.

```
var myString = "asdf";
var myNSString = new NSAttributedString (myString.Subtring (0, 2));
```

Proposed APIs:
```
public NSAttributedString (string str, int start, int length) {}
public NSMutableAttributedString (string str, int start, int length) {}
... and a ton of over overloads
```

The implementation would then just marshal part of the string, avoiding an intermediate string allocation.

Via https://github.com/xamarin/xamarin-macios/pull/5517, a new `CreateNative (string, start, offset)` overload is introduced to allow creating a native string from a managed string slice.