[Request] Options for AV1 grain synthesis

It'd be great if [AV1's grain synthesis](https://github.com/AOMediaCodec/SVT-AV1/blob/master/Docs/Appendix-Film-Grain-Synthesis.md) could get added as a UI feature. A simple number field to input the supported values of 0-50 would be great.

One currently has to add it in manually as an extra parameter: `-svtav1-params film-grain=X` ([FFmpeg docs](https://trac.ffmpeg.org/wiki/Encode/AV1#Filmgrainsynthesis))

Start by reviewing FastFlix's encoder settings UI and how extra parameters are handled. Consult the linked SVT-AV1 film-grain documentation and FFmpeg AV1 guide for the supported 0–50 values and parameter name. Done means the UI accepts the value and applies it during SVT-AV1 encoding.

Written by the indexing model from the issue text.