Allow adding "Optional System Fields" post collection creation

This is a suggestion that came up after a discussion with @benhaynes and @rijkvanzanten in Discord.

Basically, at the moment, there are "shortcuts" for commonly used fields that the user can choose upon creating a new collection. They are called "Optional System Fields".

If you however do not choose them while creating the collection, you cannot use those shortcuts later, thus if you need one, you need to create the field manually.

As @benhaynes suggested, this might instead be completely removed from the "Create Collection" workflow and instead added to the "Create Field" workflow.