Propagate opentelemetry spans across remote connections.

[rread]

The current span context is extracted and serialized with the message, and deserialized in read_request_raw() and saved in thread local storage. The generated RemoteService now creates a new span based on the message variant name, and sets the new span's parent to the extracted context.

Fixes #71

[Frando]

Thanks! The feature sounds useful.

• With this PR the wire format of a protocol changes based on feature flags for irpc. We can't do this unfortunately: You would never want your wire messages to change because some dependency enabled a feature on irpc which will - due to feature unification - affect all crates in the workspace dependency tree. So, we need a different approach here. I'll have to do some thinking how best to do this.

• The PR makes the code generation dependant on feature flags on the proc macro crate. I think this is a no-go: Due to feature unification you can never know for sure if and where a feature got enabled. So this should be based on configuration via attributes on the rpc_requests macro instead.

• The opentelemetry integration should be behind a separate feature use-tracing-opentelemetry or similar