Extend the Forgejo authentication sources to hold
instance IDs that uniquely identify a forgejo
instance. These instance IDs are used to generate
non-matching `incoming_labels` even if the user
names on different forgejo-instances are identical.
By default the disambiguation is done with an md5
hash of the forgejo authentication source URL.
The configuration allows to override this with a
custom string to keep `incoming_labels` readable.
