Allow cross-namespace references for KongPluginBindings

[mlavacca]

Problem Statement

The spec.pluginRef field contains only the name of the referred plugins, which means that at the moment, only intra-namespace references are allowed. We should add the namespace field (and make it optional) to allow cross-namespace references. Cross-namespace references should be explicitly allowed via ReferenceGrants, but that's not an API design problem.

Proposed Solution

Additional Information

Acceptance Criteria

• as a user, I can reference a plugin in a different Namespace.