Allow registering XWikiEndpointDispatchers with an additional segment

The motivation comes from the y-websocket connector that connects editors though a <baseUrl>/{roomId} pattern.
The segment of the URL just before the roomId is expected to be the role hint of the XWikiEndpointDispatcher that will handle the web socket connection.
Therefore, I propose to add a second segment endpoint configuration allowing to have one more segment.
To the best of my knowledge, there is no mechanism to have a variable part in the declared url