KVStore: Extend kvstore to retrieve all items (#50848)

* Extend kvstore to retrieve all items

* Fix comment

* Fix tests

* Change test order

* Move test outside to avoid order conditions

* Update Items to GetAll function and return a map

* Add explanation of map result

* Add description comment

Co-authored-by: Tania B <yalyna.ts@gmail.com>

pkg/services/ngalert/notifier/testing.go

@@ -201,6 +201,10 @@ func (fkv *FakeKVStore) Keys(ctx context.Context, orgID int64, namespace string,
 	return keys, nil
 }
 
+func (fkv *FakeKVStore) GetAll(ctx context.Context, orgId int64, namespace string) (map[int64]map[string]string, error) {
+	return nil, nil
+}
+
 type fakeState struct {
 	data string
 }