Private registries: Add support for Cloudsmith and GCP OIDC configurations

[mbg]

This PR primarily adds validation for Cloudsmith and GCP OIDC configurations, so that they can be correctly propagated to the authentication proxy for private package registries when given to the start-proxy action as input in GitHub-managed workflows.

The first few commits add a lightweight JSON schema implementation to simplify the specification and validation of different credential objects. The existing private registry configuration types are then migrated to make use of this.

We take advantage of this to simplify some of the validation and testing logic so that we have to update fewer places when adding new configuration types.

Then we add the two new configuration types.

Finally, we add validation for the replaces-base setting, which has been around for a while, but wasn't accepted until now.

I'd suggest reviewing this commit-by-commit.

Risk assessment

For internal use only. Please select the risk level of this change:

• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.

Which use cases does this change impact?

Workflow types:

• Managed - Impacts users with dynamic workflows (Default Setup, Code Quality, ...).

Products:

• Code Scanning - The changes impact analyses when analysis-kinds: code-scanning.
• Code Quality - The changes impact analyses when analysis-kinds: code-quality.
• Other first-party - The changes impact other first-party analyses.

Environments:

• Dotcom - Impacts CodeQL workflows on github.com and/or GitHub Enterprise Cloud with Data Residency.
• GHES - Impacts CodeQL workflows on GitHub Enterprise Server.

How did/will you validate this change?

• Unit tests - I am depending on unit test coverage (i.e. tests in .test.ts files).

If something goes wrong after this change is released, what are the mitigation and rollback strategies?

• Rollback - Change can only be disabled by rolling back the release or releasing a new version with a fix.

How will you know if something goes wrong after this change is released?

• Telemetry - I rely on existing telemetry or have made changes to the telemetry.
  • Dashboards - I will watch relevant dashboards for issues after the release. Consider whether this requires this change to be released at a particular time rather than as part of a regular release.
  • Alerts - New or existing monitors will trip if something goes wrong with this change.

Are there any special considerations for merging or releasing this change?

• No special considerations - This change can be merged at any time.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.