Consider the capabilities of other clients and server

[nesium]

We should of course consider the actual capabilities of other clients/servers. Ideally we'd simply fall back to optional values. So e.g if a Prose SDK consumer requests UserMetadata, local_time and last_activity are already optional but currently both IQs are sent regardless of the receiver's caps.

[nesium]

This also affects communication with the server, the server's MUC service and HTTP upload service. We'll need access to the stream properties in both xmpp-rs and strophe.js