[SDK] Move SFU authentication into livekit/auth and choose the endpoint by role

The OpenID flow moves to src/livekit/auth/openID.ts with the SFU config
types in a sibling file, takes an options object, and replaces the
MatrixRTC mode with the role we take on the transport plus the session's
slot ID instead of a hard-coded m.call#ROOM.

This changes behaviour: publishers on a livekit_service_url transport
always use the legacy /sfu/get endpoint, so that subscribers can predict
their user:device participant identity, and subscribers always use the
default /get_token endpoint. The MatrixRTC mode no longer influences which
endpoint is used.

On the SDK this lands as sdk/src/media-backend/livekit/auth/ with the connection map, the discriminated connection state and the client-server API flow of the following commits of element-hq/element-call#4331, which rewrite the same files:
- Model ConnectionState as a discriminated union
- Show the whole transport in debug UI and logs (SDK part)
- Let the publishing Connection authenticate itself
- Reduce ConnectionManager to a ConnectionMap
- Use the unstable LiveKit transport type
- Identify transports by server name as well as URLs
- Get SFU tokens and delegate delayed leaves through the MSC4195 CS API
- Fall back to get_token as subscriber on widget API errors
- Share a connection between homeservers using the same JWT service
- Make the auth choice mode independent again

Co-authored-by: Johannes Marbach <n0-0ne+github@mailbox.org>
(cherry picked from commit adb380cd9ed108856e42d992fc03ced8c44f1e56)

sdk/src/RTCParticipation.ts

@@ -119,8 +119,9 @@ export function createRTCParticipation(
         : `${userId}:${deviceId}`,
   };
 
-  const backend = createBackend(scope, client, options, matrixRTCMode, {
+  const backend = createBackend(scope, client, options, {
     roomId: room.roomId,
+    slotId: jsSdkSession.slotId,
     ownMembershipIdentity,
     publish: options.publish,
     encryptionSystem,
@@ -285,16 +286,10 @@ function createBackend(
   scope: ObservableScope,
   client: MatrixClient,
   { backend, mediaQuality }: RTCParticipationOptions,
-  matrixRTCMode: MatrixRTCMode,
   context: MediaBackendContext,
 ): MediaBackend {
   if (backend) return backend(scope, context);
-  return createLivekitBackend(scope, context, {
-    client,
-    mediaQuality,
-    tokenEndpoint:
-      matrixRTCMode === MatrixRTCMode.Matrix_2_0 ? "msc4195" : "legacy",
-  });
+  return createLivekitBackend(scope, context, { client, mediaQuality });
 }
 
 /** The slot's per-participant keys: the ones it already holds on subscribe, then each new one. */