Make an HBFont from a CTFontRef.

By including ```rive/text/font_hb.hpp``` in iOS you'll get access to ```HBFont::FromSystem``` along with the ```HBFont::Decode``` (which we've been using so far to generate Font objects to put into the fallback list).

You can get a UIFont with [this API](https://developer.apple.com/documentation/uikit/uifont/1619027-systemfontofsize?language=objc).

You should be able to cast that to a CTFontRef:
```
CTFontRef ctFont = (__bridge CTFontRef)uiFont;
auto fallbackFont = HBFont::FromSystem((void*)ctFont);
```

You can then use that in a fallback font procedure (like the example from the editor below) to load on demand/cache/provide from a list etc:
https://github.com/rive-app/rive/blob/b5b930f88f18280afa44683c5756066abaa9b1dc/packages/rive_common/macos/rive_text/rive_text.cpp#L153-L175

Diffs=
da1bb7745 Make an HBFont from a CTFontRef. (#7661)

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