Generate Code for Adding a Custom Field in a Plugin

[shyim]

Currently we mostly create custom fields via admin panel. But to properly deploy a custom module, the field(s) should be created in code.

It would be nice if the frosh development helper could support creating such custom fields, like described in

https://developer.shopware.com/docs/guides/plugins/plugins/framework/custom-field/add-custom-field#add-column-in-database-table

taken from: FriendsOfShopware/FroshDevelopmentHelper#40