Physics bridge: Low frequency upload of transforms to server

[jabenk]

At the moment the server only routes the transforms from the clients and does not read them (binary format).
In addition, later the physics bridge might stream via a different server or directly peer to peer.

So we need to create a low-frequency upload channel from each client to the server to update the transforms.

[jabenk]

microsoft/mixed-reality-extension-unity#204
#642