Possibility to configure filterable attributes from UI

[einorler]

Currently the plugin has the attribute_whitelist parameter in configuration. This is not exactly the best implementation as this denies non-technical administrators of the shop from adding additional attributes for the shop to filter on.
I propose the introduction of filterable field in ProductModel entity. Me and my team have done the same in our SQL based filter plugin. This way the administrator of the shop can simply flag this option in admin panel and from that moment on these product attributes will be taken into the ES index.
I think I could even find time to make a contribution on this, if you consider it a valid improvement