Storage Schema endpoints feature flag

Related ADR: 62

Background Instead of complete irreversible removal of the Storage Schema endpoints, a fail-safe option to restore application functionality is needed in cases where missed dependencies on Storage Schemas APIs have gone undetected. The remediation for application failures could be heavy-duty if schemas need to be migrated, thus increasing the need for at least a read-only ability on Storage Schemas to be available for the duration of the remediation.
This MR provides:

• The described fail-safe capability in case of application failure
• Controlled shutoffs of the Storage Schema (hopefully proactively) in order to identify non-conforming or failing client apps.