Added support for generic font family parameters

Hello, I discovered resvg and this project recently and it would fit very well for the problem I am working on.

But like it is done in resvg CLI, I would need to specify the font for a generic font family when rendering SVG content.

You can see those options for usvg being used here.

That's 5 possible values that are ultimately used to setup fontdb and set the font for the generic font families:

serif
sans-serif
cursive
fantasy
monospace

So I added those parameters to the ResvgSharp options and use them to setup fontdb in the Rust wrapper, like it's done in usvg.

Thank you