feat: add separate endpoint to fetch function names for autocomplete (#12840)

* WIP

* Add unit test for API

* Add spec

* Fix unit test

* Fix unit test

* Fix test

* Fix test

* Add period to error message

superset-frontend/src/SqlLab/components/SqlEditorLeftBar.jsx

@@ -80,6 +80,10 @@ export default class SqlEditorLeftBar extends React.PureComponent {
 
   onDbChange(db) {
     this.props.actions.queryEditorSetDb(this.props.queryEditor, db.id);
+    this.props.actions.queryEditorSetFunctionNames(
+      this.props.queryEditor,
+      db.id,
+    );
   }
 
   onTableChange(tableName, schemaName) {