Additional LDAP Authentication Methods (#1287)

This patch allows anonymous and user based authentication to LDAP
servers. This change is based on a patch against bn-ldap-authentication:

-  https://github.com/blindsidenetworks/bn-ldap-authentication/pull/2

The patch introduces a new environment variable `LDAP_AUTH` which
controls the authentication method used against the LDAP server:

- `anonymous` enables an anonymous bind to the LDAP with no password
  being used.

- `user` uses the user's own credentials to search for his data,
  enabling authenticated login to LDAP without the need for a user with
  global read privileges.

The default still remains at using a bind user, allowing for a seamless
upgrade path from the previous version.

This fixes #1082

Co-authored-by: Jesus Federico <jesus@123it.ca>

sample.env

@@ -61,6 +61,7 @@ OAUTH2_REDIRECT=
 #   LDAP_METHOD=plain
 #   LDAP_UID=uid
 #   LDAP_BASE=dc=example,dc=com
+#   LDAP_AUTH=simple
 #   LDAP_BIND_DN=cn=admin,dc=example,dc=com
 #   LDAP_PASSWORD=password
 #   LDAP_ROLE_FIELD=ou
@@ -70,6 +71,7 @@ LDAP_METHOD=
 LDAP_UID=
 LDAP_BASE=
 LDAP_BIND_DN=
+LDAP_AUTH=
 LDAP_PASSWORD=
 LDAP_ROLE_FIELD=