TTML: Captions - Support for style tts:textOutline

Searching
https://github.com/google/ExoPlayer/labels/enhancement
and
https://github.com/google/ExoPlayer/pulls
I cannot see "outline" or "textOutline" mentioned.

### TTML: Support for tts:textOutline
The https://exoplayer.dev/dash.html indicates support for ISO/IEC 14496-30 which in turn references https://www.w3.org/TR/ttml1/#style-attribute-textOutline.
This is missing from TtmlDecoder parseStyleAttributes
This is not supported and is very useful for high-lighting white subtitle text against a white background. (Our customers have complained)

### Proposed solution
Implement support for https://www.w3.org/TR/2018/REC-ttml2-20181108/#style-attribute-textOutline as per the example in the document:
``

### Alternatives considered
Possible to over-ride the supplied subtites with CaptionStyleCompat but this is not suitable for all customers and it is best to respect the in-stream track.