Add runner disk info and image version to status report - #1825

This PR adds three new fields to the status report:

• Available disk space on the runner
• Total disk space on the runner
• Image version of the runner, for GitHub-hosted runners

We also log a warning if there is less than 2 GB of disk space available at various points in the run. For example, we log a warning if there is less than 2 GB of disk available at the start of init, but not at the end of analyze as the user is unlikely to need much space for the rest of the job. For neatness, we only log the warning once per run, but we continue logging the amount of free disk space in the debug logs.

Commit-by-commit review recommended since we do some refactoring, e.g. pulling out status reporting into its own file.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.