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/ldap.go

@@ -32,6 +32,9 @@ type ProviderInstance struct {
 	searchAllowedGroups []*strfmt.UUID
 	boundUsersMutex     sync.RWMutex
 	boundUsers          map[string]UserFlags
+
+	uidStartNumber int32
+	gidStartNumber int32
 }
 
 type UserFlags struct {
@@ -47,6 +50,17 @@ type LDAPServer struct {
 	providers   []*ProviderInstance
 }
 
+type LDAPGroup struct {
+	dn string
+	cn string
+	uid string
+	gidNumber string
+	member []string
+	isSuperuser bool
+	isVirtualGroup bool
+	akAttributes interface{}
+}
+
 func NewServer(ac *ak.APIController) *LDAPServer {
 	s := ldap.NewServer()
 	s.EnforceLDAP = true