[Feature request] Support font resources for SpannableBuilder

[eagskunst]

I think one of the main use case sof the SpannableBuilder is to build a single TextView with multiple fonts. It would be cool if this can be done with the SpannableBuilder.

I will like to contribute to this but don't know exactly where to start 🤔.

In my app I'm currently loading fonts with ResourcesCompat.getFont() and a custom @attr function.