Support retrieving video image by frame index on VideoFrameDecoder

[tnj]

Is your feature request related to a problem? Please describe.
There is no way to decode an image by its frame index using coil-video.

MediaMetadataRetriever has getFrameAtIndex that is added on API 28, but it is not implemented on VideoFrameDecoder yet.

Describe the solution you'd like
Add a video frame index option to VideoFrameDecoder in addition to the current videoFrameMillis, videoFrameMicros and videoFramePercent

Additional context
I've implemented this for my project main...tnj:coil:video-index-support

[colinrtwhite]

Thanks, your branch looks good. Feel free to submit it as a PR!