feat(policy): Proto - root certificates by namespace (#2800)

### Proposed Changes

This pull request adds support for managing certificates within
namespaces in the policy API and documentation. It introduces new data
models, API endpoints, and documentation for associating certificates
with namespaces, including assigning and removing certificates, and
listing root certificates as part of a namespace's properties.

- Introduced AssignCertificateToNamespace and
RemoveCertificateFromNamespace RPCs to NamespaceService.
- Defined supporting protobuf messages, schemas, and service handlers.
- Updated OpenAPI documentation to include the new APIs.
- Extended namespaces with root certificate support.

### Checklist

- [ ] I have added or updated unit tests
- [ ] I have added or updated integration tests (if appropriate)
- [x] I have added or updated documentation

### Testing Instructions

Author: pflynn-virtru