Add Pre submit client tests iwth local trusted root and signing config

[ramonpetgrave64]

Summary

Addresses #269

Adds PR tests against the clients that can accept a local trusted_root.json and singing_config.json, or trust_config.json (python and sigstore-go, so far) against the PR's copy of those files. This ensures compatibility with file layouts, key types, hosts, etc.

Currently the tests run against the latest release of those clients.

• The sigstore-go test is currently failing because it's not yet using the new version of protobuf-specs that include the operator field.

  • https://github.com/sigstore/root-signing-staging/actions/runs/15569497275/job/43841741611?pr=278#step:5:18

• A maintainer probably needs to approve the run to allow id-token: write. These changes are working on my fork.

  • https://github.com/sigstore/root-signing-staging/actions/runs/15569497275/job/43841741596#step:6:110

Release Note

• Adds PR tests against the clients that can accept a local trusted_root.json and singing_config.json, or trust_config.json (python and sigstore-go, so far) against the PR's copy of those files.

Documentation

None