feat(server/player): support getting/setting nested metadata

[mafewtm]

Description

Handles nested metadata as requested in #646. Doesn't allow for more than one level deep. I think there's probably a better way to do this but right now it just checks for periods as seperators.

Checklist

• I have personally loaded this code into an updated Qbox project and checked all of its functionality.
• My pull request fits the contribution guidelines & code conventions.

[Manason]

What if we split the key on periods using gmatch, then iterate over the nested keys to have this solution work for any depth. One thing I didn't think of, could this be a problem for existing metadata event handlers. i.e. if someone is listening for a change in the key 'license', then this solution would also need to emit the event for the top level key in addition to the nested key string to preserve backwards compatibility.