Support recursive exploration of folders

The MR adds the following flags which are needed to address https://gitlab.developers.cam.ac.uk/uis/devops/infra/sql-backup/-/issues/2:

• --dry-run makes testing future code changes easier by providing a "read-only" option which skips performing backups.
• --recursive enables recursive search for projects in sub-folders
• --exclude-folder to exclude certain folders from the recursive search

Together these allow us to recurse into the "DevOps" folder in Google but exclude projects within the "Experiments" folder.

You can see this in action (along with unrelated Cloud IAM weirdness) at https://gitlab.developers.cam.ac.uk/uis/devops/infra/sql-backup/-/jobs/87838.

The Cloud IAM errors appear to be caused by a delay in applying IAM policies causing stale patches to be applied post bucket creation. They are annoying but unrelated to this MR.

Closes #2 (closed)