[wip] `jhack unravel` command

[PietroPasotti]

This adds a new command to find and display the relation graph starting from a single juju application. It supports in- and cross-model relations.

Todo:

• plotting mode, outputting a graphically pleasing representation
• run tests against the graph e.g. 'each grafana-agent charm is related to at least a loki-k8s charm'