Make REST Catalog Namespace Separator Configurable

### Feature Request / Improvement

Similar to the proposed changes in Java/ REST Catalog Open API Spec, we'd like to make the REST Catalog Namespace Separator Configurable address issued in specific catalog implementations.

Related PRs:
https://github.com/apache/iceberg/pull/10877
https://github.com/apache/iceberg/pull/10904
https://github.com/apache/iceberg/pull/10905