examples: webrtc: rust: i64 -> u64 for session and handle ids

I'm implementing a Janus Video Room plugin in gst-plugins-rs and saw that the example here is using i64 for the session and handle ids. Taking a look in the janus-gateway source code, they're u64s:

• https://github.com/meetecho/janus-gateway/blob/359706e54f3e9588a469e194ef3472099753efb2/src/janus.h#L46
• https://github.com/meetecho/janus-gateway/blob/359706e54f3e9588a469e194ef3472099753efb2/src/sctp.h#L109
• https://github.com/meetecho/janus-gateway/blob/359706e54f3e9588a469e194ef3472099753efb2/src/ice.h#L352