Media: allow storing the last playback speed

When storing and recovering the last playback state, it would be nice to also have the option to recover the last used playback speed. So far, the data model lacks the needed property.

added User Feedback feature good-first-issue labels

marked this issue as related to vlc-ios#1739

mentioned in issue vlc-ios#806

Since this is a simple value placement in the DB and that the medialibrary won't interact at all with it, I suggest using the Media generic metadata API for this.
Please refer to the android port for some example: https://code.videolan.org/videolan/vlc-android/-/blob/master/medialibrary/src/org/videolan/medialibrary/interfaces/media/MediaWrapper.java?ref_type=heads#L64