Add a GCS deploy workflow

[jku]

Fixes #7

This uploads the repository to GCS and invalidates the CDN cache.

• the workflow is a custom one for root-signing
• it is called as a reusable workflow from the TUF-on-CI publish workflow (so gets automated issue failing on errors)

GCP configuration is mostly hard coded, only service account details are inputs:

• service account: same as online signing
• project: projectsigstore-staging
• GCS bucket: tuf-root-staging
• CDN load balancer: tuf-repo-cdn-lb

[jku]

Comparison to current implementation

See https://github.com/sigstore/root-signing/blob/main/.github/workflows/sync-main-to-staging.yml

The main logical change is that the git repository now only contains the current metadata versions. This means that if the publishing mechanism wants to both delete old metadata and artifacts but also ensure old metadata & artifacts are available for clients to use for a period of time, it needs to manage that. In practice there should be no pressing need for a sigstore repository to delete old metadata & artifacts: there will only be small amount of new files per year in normal operation.

The technical changes are listed below:

• runs as part of Publish workflow
• Removed gcloud auth login: I don't know what the purpose was
• Removed create_credentials_file: true from google-github-actions/auth: this is the default
• Simplified special case for timestamp.json (it should be uploaded after all the other files)
• Removed expiry checks: Publish workflow has already run a client smoke test that fails on expiry
• Removed deletion of files from bucket: Some form of this can be re-implemented later but there should be no major need for it for decades (considering the usage pattern)
• Removed failure handling: this is provided by Publish workflow that is the caller of this reusable workflow

[jku]

@haydentherapper feel free to have a look already, I think this should be finished (but let's not merge yet).

[jku]

Two obvious feature requests:

• Automated testing for the GCS repository: I think this can be a separate feature
• Use a GitHub deployment environment: This would allow us to control the final deployment with release reviews etc. We probably don't want more bureaucracy for staging so I don't think we need this right now (but this might be a separate feature to implement before production is moved to tuf-on-ci)

[jku]

Rebased on main.

I'm marking this ready:

• Tests pass on current repository, I've also tested manually with sigstore-python
• Other signers have now been added (so my yubikey is no longer critical infra)
• This PR has no release gating: both timestamps and other changes get published automatically once they pass the client tests. I believe this is fine for staging: we can work on #54 afterwards
• quick risk analysis:
  • complete upload failure is not really an issue: currently published timestamp remains valid ~forever
  • the new timestamp is set to expire in 7 days so succeeding once and then failing later would be more of an issue
  • In general, we'll have ~4 days (length of the signing period) to resolve issues in staging. This is similar to production