Alerting: improve alerting default datasource search when extracting alerts (#29993)

* improve alerting search datasource

* correct the xorm get usage

* adding default datasource unittest

pkg/models/datasource.go

@@ -208,6 +208,12 @@ type GetDataSourcesQuery struct {
 	Result          []*DataSource
 }
 
+type GetDefaultDataSourceQuery struct {
+	OrgId  int64
+	User   *SignedInUser
+	Result *DataSource
+}
+
 type GetDataSourceByIdQuery struct {
 	Id     int64
 	OrgId  int64