#471 added initial support for ReferenceGrants by allowing Gateways to reference Secrets in a different namespace. The gateway controller adds the ReferenceGrant and the ReferenceGrant namespace to the provider watchable map when:

1. A Gateway listener references a Secret in a different namespace.
2. A ReferenceGrant in the above namespace exists and permits the Gateways from the Gateway's namespace to Secrets.

When the Gateway is deleted, the ReferenceGrant and ReferenceGrant namespace should be deleted from the provider's watchable map if no other Gateway references the ReferenceGrant.