Added CI to create PRs to bump to latest Ark timestamps

[wtripp180901]

Added a nightly workflow which will check the URLs defined in appliances_repo_timestamp_sources for newer timestamps, update appliances_repo_timestamps and create a PR if any timestamps have been updated

[sjpb]

See discussion on slack re. simplifying things so we're not defining repo URLs in 3x places.

Also needs to
a) kick off an image build in the PR - and not run the stackhpc.yml workflow

• Could do this by changing that workflow to not run on branch names like e.g. auto/ and setting that branch for the PR
  b) make a commit to the PR to change environments/.stackhpc/terraform/cluster* vars to the new image - then run the stackhpc workflow

OR

• do the image build on the branch
• update ../cluster* definition
• THEN open the PR (which runs stackhpc.yml)

[wtripp180901]

Todo:

• Check fatimage dispatch outside of update-timestamps still works as intended (DONE: checks out dispatched branch correctly https://github.com/stackhpc/ansible-slurm-appliance/actions/runs/12669541401)
• Check CI still runs on correct branch outside of update-timestamps (DONE: https://github.com/stackhpc/ansible-slurm-appliance/actions/runs/12670021897)

Todo after review but before merging:

• Change reference for is_callee action to main (will break the workflow here but will work once merged)
• Remove on.pull_request and on.workflow_dispatch from update_timestamps workflow
• Delete current auto/bump-timestamps branch (it won't contain the two changes above)

[wtripp180901]

successful e2e test https://github.com/stackhpc/ansible-slurm-appliance/actions/runs/12670021951