[Runtime] Support player pose synchronization (or rather, subscription to specific transforms from an actor)

[eanders-ms]

Not all platforms have the capability to synchronize player skeletons or other data. Add an option to the runtime for synchronizing data between clients. A generic p2p rpc, essentially. This would enable clients to sync skeleton updates.

[eanders-ms]

Rationale: I can't test multiplayer attachments in the test bed.

[eanders-ms]

Curious how this would work in a hybrid environment. The ability to sync attachments between Altspace and the testbed would be really cool, and pave the way to support other hybrid configurations. Perhaps we could define a generic avatar skeleton (would have to work for non-avatar platforms too), and platforms could choose to sync this or not.

[sorenhan]

I don't think this should be implemented as suggested. IMO host apps are responsible for general pose synchronization. There are much more optimzied ways to do this than the MRE SDK.

I think synchronization only in one specific case (see #495) If this proposal is for the MRE to expose a way to subscribe to specific transforms then that's serves a purpose

I think it make sense to do this the same way an MRE can subscribe to specific actor transforms