Add search bind support, fix group_identifier typo, fix gidNumber query

- Add search_bind_enabled/search_bind_dn/search_bind_password config
- Support both group_indentifier (original typo) and group_identifier
- Skip gidNumber query when attribute is empty
- Use searchLdap for all search/group queries

login-ldap.yaml

@@ -5,6 +5,9 @@ version: 3
 ssl: false
 start_tls: false
 opt_referrals: false
+search_bind_enabled: false
+search_bind_dn:
+search_bind_password:
 user_dn: 'uid=[username],dc=company,dc=com'
 search_dn:
 group_dn: