Create a "data" field using a map data type.

Profiles have versatile use. Sometimes we may need to store arbitrary data about them that does not quite require a field. An example is in #3022850: [Addressbook, part 1] Rework the ownership model for customer profiles. We want to maintain a "source" value to know how a profile was derived from another profile, and if it should be converted to a user's addressbook.