providers/ldap: Added auto-generated uidNumber and guidNumber generated attributes for use with SSSD and similar software. (#1138)

* Added auto-generated uidNumber and guidNumber generated attributes for
use with SSSD and similar software.

The starting number for uid/gid can be configured iva environtment
variables and is by default 2000 which should work fine for most instances unless there are more than
999 local accounts on the server/computer.

The uidNumber is just the users Pk + the starting number.
The guidNumber is calculated by the last couple of bytes in the uuid of
the group + the starting number, this should have a low enough chance
for collisions that it's going to be fine for most use cases.

I have not added any interface stuff for configuring the environment variables as I couldn't really find my way around all the places I'd have to edit to add it and the default values should in my opinion be fine for 99% use cases.

* Add a 'fake' primary group for each user

* First attempt att adding config to interface

* Updated API to support new fields

* Refactor code, update documentation and remove obsolete comment

Simplify `GetRIDForGroup`, was a bit overcomplicated before.

Add an additional class/struct `LDAPGroup` which is the new argument
for `pi.GroupEntry` and util functions to create `LDAPGroup` from api.Group and api.User

Add proper support in the interface for changing gidNumber and uidNumber starting points

* make lint-fix for the migration files

outpost/pkg/ldap/api.go

@@ -37,8 +37,10 @@ func (ls *LDAPServer) Refresh() error {
 			boundUsersMutex:     sync.RWMutex{},
 			boundUsers:          make(map[string]UserFlags),
 			s:                   ls,
-			log:                 logger,
+			log:                 log.WithField("logger", "authentik.outpost.ldap").WithField("provider", provider.Name),
 			tlsServerName:       provider.TlsServerName,
+			uidStartNumber:		 *provider.UidStartNumber,
+			gidStartNumber:		 *provider.GidStartNumber,
 		}
 		if provider.Certificate.Get() != nil {
 			logger.WithField("provider", provider.Name).Debug("Enabling TLS")