Add Modifiable Metadata Strings to the Commit Info

## View & Edit Metadata
In combination with the content promotion from [PR #12](https://github.com/AP-Sensing/ostree-tui/pull/12), it would be useful to be able to **view & edit** commit metadata.

## Implementation
This would fit right below the existing commit info. Editing the existing commit info should also be part of the functionality (and probably is the way bigger change).