registration: Allow custom prefixes for configuration settings

Allow extensions which are using "$eg" or any other prefix to migrate.

Extensions can override the default of "wg" by setting a magic "_prefix"
key in the "config" object.

Note that the migration helper script will not be able to automatically
migrate custom-prefixed configuration settings.

Bug: T97186
Change-Id: I79203cd5e3a2405b92ad01da869de3bd3d359d19