feat(core): new ExtSource type for IT4I

- Used for group structure synchronization only.
- Supports only lightweight sync!
- Group members extsource must be set to "PERUN" SQL ext source
  as provided einfra logins from IT4I are converted to perun
  users IDs in a safe manner skipping any inconsistencies
  (e.g. anonymized accounts).
  This prevents unnecessary creation of new UserExtSource for
  all users plus we handle any possible inconsistencies
  in existing data.

Author: zlamalp