MySQL, Postgres, MSSQL: Fix validating query with template variables in alert (#19237)

Adds support for validating query in alert for mysql, 
postgres and mssql.

Fixes #13155

public/app/plugins/datasource/postgres/datasource.ts

@@ -160,4 +160,19 @@ export class PostgresDatasource {
         }
       });
   }
+
+  targetContainsTemplate(target: any) {
+    let rawSql = '';
+
+    if (target.rawQuery) {
+      rawSql = target.rawSql;
+    } else {
+      const query = new PostgresQuery(target);
+      rawSql = query.buildQuery();
+    }
+
+    rawSql = rawSql.replace('$__', '');
+
+    return this.templateSrv.variableExists(rawSql);
+  }
 }