[Feature Request] Support for using custom fonts

[kryptonit1]

Is your feature request related to a problem? Please describe.
It would be nice if it was possible to use another font for title, subtitles and body (onboarding UI). As a company/organization you might have a graphic profile with specific fonts that should be used.

Describe the solution you'd like
The possibility to use custom fonts in IBM Notifier without using the HTML accessory view for example.

Describe alternatives you've considered
I know that this is possible using the HTML accessory view, but it would be much easier to be able to specify another font to use for the UI:s using a parameter for example. The code needed would be much less.

It is also currently not possible to have an HTML and Image accessory view stacked on top of each other (at least I do not think so), which means that if I try to achieve this with an HTML accessory view I will have to accept that the Image accessory view will appear next to the HTML accessory view instead of below. So the end result will not be the same.

Additional context
N/A