Add support for sending the RP-initiated logout request as POST instead of GET

Currently, the RP-initiated logout is done through a redirect (so a GET), but it would be interesting to also allow configuring the authenticator to send POST request instead (as allowed by the specification).

In terms of implementation, it probably means redirecting to a dedicate XWiki endpoint which tend use javascript to send the POST request to the provider, instead of directly redirecting to the provider's logout endpoint.