Add an ability to fetch user's membership from
  groups of a different type by specifying multiple
  group attribute to user attribute value matchers
  in the Dex config:

    userMatchers:
    - userAttr: uid
      groupAttr: memberUid
    - userAttr: DN
      groupAttr: member

  In other words the user's groups can be fetched now from
  ldap structure similar to the following:

    dn: cn=john,ou=People,dc=example,dc=org
    objectClass: person
    objectClass: inetOrgPerson
    sn: doe
    cn: john
    uid: johndoe
    mail: johndoe@example.com
    userpassword: bar

    dn: cn=qa,ou=Groups,ou=Portland,dc=example,dc=org
    objectClass: groupOfNames
    cn: qa
    member: cn=john,ou=People,dc=example,dc=org

    dn: cn=logger,ou=UnixGroups,ou=Portland,dc=example,dc=org
    objectClass: posixGroup
    gidNumber: 1000
    cn: logger
    memberUid: johndoe

Signed-off-by: Vitaliy Dmitriev <vi7alya@gmail.com>