Alerting: Update GetRuleGroupAlertRules to accept optional rule group (#46889)

* rename GetRuleGroupAlertRules to GetAlertRules
* make rule group optional in GetAlertRulesQuery
* simplify FakeStore. the current structure did not support optional rule group

pkg/services/ngalert/schedule/schedule_unit_test.go

@@ -1117,12 +1117,12 @@ func CreateTestAlertRule(t *testing.T, dbstore *store.FakeRuleStore, intervalSec
 	})
 	require.NoError(t, err)
 
-	q := models.ListRuleGroupAlertRulesQuery{
+	q := models.GetAlertRulesQuery{
 		OrgID:        orgID,
 		NamespaceUID: "namespace",
-		RuleGroup:    ruleGroup,
+		RuleGroup:    &ruleGroup,
 	}
-	err = dbstore.GetRuleGroupAlertRules(ctx, &q)
+	err = dbstore.GetAlertRules(ctx, &q)
 	require.NoError(t, err)
 	require.NotEmpty(t, q.Result)