Early work for lp1951415. containeragent to pebble.

[tlm]

Early work for lp1951415 that allows the container agent to run under pebble.

As part of lp1951415 we need to move the container agent under pebble. This PR introduces that change.

Checklist

• [x] Code style: imports ordered, good names, simple structure, etc
• [x] Comments saying why design decisions were made
• [x] Go unit tests, with comments saying what you're testing
• [x] Integration tests, with comments saying what you're testing
• [x] doc.go added or updated in changed packages

QA steps

1. Bootstrap Juju to a Kubernetes cluster
2. Deploy a workload and check that the container agent for each pod is running under pebble and happy.
3. Check relations.

Documentation changes

N/A

Bug reference

https://bugs.launchpad.net/juju/+bug/1951415