feat: add --instance support for TUF-based trust bootstrapping

[SequeI]

Summary

Allow users to bootstrap and use a Sigstore instance by its TUF repository URL instead of requiring a full ClientTrustConfig JSON file. Adds a trust-instance CLI command to seed the local TUF cache from a root.json, and an --instance option on sign/verify to resolve trust configuration via TUF at runtime. Bumps sigstore dependency to >=4.2.

# Fetch the public goods root.json
curl -o /tmp/root.json https://tuf-repo-cdn.sigstore.dev/root.json

# Bootstrap trust for the instance
model_signing trust-instance --instance https://tuf-repo-cdn.sigstore.dev /tmp/root.json

# Sign the model
model_signing sign --instance https://tuf-repo-cdn.sigstore.dev --signature /tmp/test-model.sig bert_based_uncased

# Verify the model
model_signing verify sigstore --instance https://tuf-repo-cdn.sigstore.dev \
--signature /tmp/test-model.sig \
--identity $EMAIL \
--identity-provider https://accounts.google.com \
bert_based_uncased

Also tested with personal private RH sigstore instance, worked perfect.

Closes #618

Checklist

• All commits are signed-off, using DCO
• All new code has docstrings and type annotations
• All new code is covered by tests. Aim for at least 90% coverage. CI is configured to highlight lines not covered by tests.
• Public facing changes are paired with documentation changes
• Release note has been added to CHANGELOG.md if needed

[SequeI]

Tests fail rn because of the OIDC pulling action, I noticed it seems to fail around this time period I opened the PR at. Will rerun tests in a different time period.

[mihaimaruseac]

Should we use instead the tooling that already exists in the Sigstore ecosystem for this? I'm wary of creating divergences in the future.

CC @Hayden-IO as this also relates to the way we interoperate within the wider Sigstore ecosystem

[Hayden-IO]

This is a necessary feature to be able to support private deployments, so I'm supportive of it.

I don't love add model_signing trust-instance, but I'm not sure another way around it that wouldn't involve downloading and using Cosgin. Though, you could also say that sign is sufficient to cache the trust root, it's just the first time you call it, it does an extra call.