Add a provisioning comment field

This request is to add a `comment` or similar field to the provisioning section of a job request. The purpose of this field is to allow passing a message about the job which would then be visible from the WebUI. The primary use case is to provide a way to track jobs created by automation back to the automation which created it. for example:

```
job_queue: my-favorite-queue
provision_data:
distro: focal
comment: "Provisioned by primus team. url: https://automation.url/test-runner"

test_data:
test_cmds: |
echo "run a test"
```

This would then show up under the `Provision Data` for the job url https://testflinger.canonical.com/jobs/job_id:
```
{'distro': 'focal', 'comment': 'Provisioned by primus team. url: https://automation.url/test-runner'}
```

Currently, we do this by adding content to the `test_cmds` to echo out the same kind of information to a `provisioned_by` file. But with recent UI changes, having an explicit field for this would be helpful.

Start by tracing how provisioning data is accepted for a job request and how the WebUI renders the Provision Data section. Add support for an explicit comment value and verify that it is preserved and displayed for jobs created with the example configuration.

Written by the indexing model from the issue text.