LDAP auth: indroduce config option 'ldap_user_attribute'

This option gives us
- flexible authentication options where the name used for logging on
  does not have to be the account name
  e.g. use ldap_filter = (&(obhjectclass=inetOrgperson)(|(cn={0]})(mail={0})))
  to allow loginng on using the cn or the mail address
- automatically consistent / canonicalized username values
  (i.e. exactly the way the LDAP server returns them)

radicale/config.py

@@ -227,6 +227,10 @@ DEFAULT_CONFIG_SCHEMA: types.CONFIG_SCHEMA = OrderedDict([
             "value": "(cn={0})",
             "help": "the search filter to find the user DN to authenticate by the username",
             "type": str}),
+        ("ldap_user_attribute", {
+            "value": "",
+            "help": "the attribute to be used as username after authentication",
+            "type": str}),
         ("ldap_load_groups", {
             "value": "False",
             "help": "load the ldap groups of the authenticated user",