Placeholder Plugin Providers for Repos and added testPlugin struct (#1532)

* Create minimalistic plugin map and test plugin.

* Pass secrets and attributes to the plugin when creating catalogs or sets.  Add tests for CreateSet and CreateCatalog repo methods.

* make fmt and extract out getPrivateCatalog and getCatalog helper methods.

* Exporting TestPluginServer so it can be used in the handlers.

* Keep separate persisted data from host catalogs lookups which facilitate passing it onto the plugins.

* Add TODO to merge get Catalog and get Persisted data.

* Adding tests for unimplemented being returned by the plugin.

* Use GetFields() instead of AsMap() to confirm attributes should be populated. Make clone consistent between catalog and sets.

Co-authored-by: Jeff Mitchell <jeffrey.mitchell@gmail.com>

internal/proto/local/controller/storage/host/plugin/store/v1/host.proto

@@ -43,7 +43,7 @@ message HostCatalog {
   uint32 version = 8;
 
   // attributes is a jsonb formatted field.
-  // @inject_tag: `gorm:"default:null"`
+  // @inject_tag: `gorm:"not_null"`
   bytes attributes = 9;
 }
 
@@ -79,7 +79,7 @@ message HostSet {
   uint32 version = 7;
 
   // attributes is a byte field containing marshaled JSON data
-  // @inject_tag: `gorm:"default:null"`
+  // @inject_tag: `gorm:"not_null"`
   bytes attributes = 8;
 
   // preferred_endpoints stores string preference values