We need to manage *tracks* from `charmcraft`. This involves two situations: - the tracks available for a given charm: - new command: `tracks <charm-name>` - this should be available from the Store already, as part of the response of `list_releases` - set the default track for a given charm: - new command: `set-default-track <charm-name> <track-name>` - the Store needs to provide the proper API for this Related: spec for [Snapcraft tracks](https://github.com/snapcore/snapcraft/blob/master/specifications/default-tracks.org).