Allow for arbitrary additions to user profile

Alternatively to coding new fields for everything, allow users to add fields along with what service they are associated with.

No file, test, or entry point is named. Start by locating the plugin's user-profile extension and how existing rel-me and h-card fields are stored and rendered. Before coding, clarify the arbitrary field data model, associated service values, profile editing behavior, and acceptance criteria.

Written by the indexing model from the issue text.