make the user picker extensible

In the context of ActivityPub, we want to be able to use the user picker to suggest users from the fediverse.
In addition, we want to be able to use the user picker to suggest users, groups, and users from the fediverse when using the mention macro.

Hence, it is interesting to allow the user picker to propose results from different sources of groups or users, and to allow these sources to be contributed to by extension.

see https://forum.xwiki.org/t/extensible-user-picker/7537