Add "Auto" option to Mapbox visualization point radius (#7579)

* Add Auto option to point radius to make it work as expected since the example using this didn't work because of this (and also as the tooltip says the auto option should be available)

* Remove trailing space

superset/assets/src/explore/controls.jsx

@@ -1849,7 +1849,7 @@ export const controls = {
     'Either a numerical column or `Auto`, which scales the point based ' +
     'on the largest cluster'),
     mapStateToProps: state => ({
-      choices: columnChoices(state.datasource),
+      choices: formatSelectOptions(['Auto']).concat(columnChoices(state.datasource)),
     }),
   },