[tunnelbroker] Add support for unauthenticated websocket sessions

Summary:
Resolves ENG-4381.
Added tunnelbroker support for starting anonymous sessions, which omit the VerifyUserAccessToken RPC cal RPC call.
As for now, the anonymous sessions can only listen for messages, and cannot send any.

Depends on D10779.

Test Plan:

• Modified client code to send AnonymousInitializationMessage instead of ConnectionInitializationMessage
• Verified that websocket session is started
• Verified that the device can receive messages
• Verified that sending a message from anonymous session results in receiving MessageSentStatus::Unauthenticated error.

Reviewers: kamil, michal

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10782