tuf-on-ci migration preparation

[jku]

This is still a draft, missing a few bits and a self-review still.

Prepare for migration to tuf-on-ci managed TUF repository

This is a bunch of separate changes that prepare for #929 . They are in a single PR since they should all be no-ops with regards to day-to-day operations for now: just laying out some foundations. I can split to separate PRs if there's demand for that. See individual commits for more rationale.

• [x] playbooks for maintainers and signers
• [x] Update targets/ content to match what repository artifacts should be
• [x] rename current test.yml to ci-test.yml (because of naming conflict)
• [x] Add tuf-on-ci workflows in a disabled state
• [x] Add metadata whitespace modifying script to use during import
• [ ] Add migration playbook
• [ ] Add import configuration for tuf-on-ci

This is a part of #1247

[jku]

This is likely ready for first review... but it's big enough that I should really split it first. I will do that tomorrow

[jku]

closing, I've split this into three separate PRs