[delegations] Create a reusable workflow that can create a delegations metadata file and sign

[asraa]

Description

Consume this workflow in an external repository.

Ensure that this workflow can

1. Checkout a configured repository & branch (root-signing and the ceremony branch)
2. Create the metadata file (or update)
3. Sign with a key
4. Create a PR with the updated the delegation file against the configured repository & branch

cc @kommendorkapten

[asraa]

Closed, because will be handled by the delegation

[jku]

this should be decently handled by tuf-on-ci-delegate (to create the delegation) and theupdateframework/tuf-on-ci/actions/online-sign-targets action for signing in delegated metadata in a workflow