Add context to RPC impl on the server side

[minghuaw]

The context could be used for

• cascading cancellation (Is the project still in active development? #24)
• access client connection info (ip/node name, etc.) Error on server compile #27
• distributed tracing (migrate to tracing and distributed tracing with opentelemetry #26)

[minghuaw]

Proposed Usage

A new type Context will be added, which (for now) will contain information such as client connection info and distributed tracing (if enabled). The proposed usage looks like below

#[export_impl]
impl Service for State {
  #[export_method]
  pub async fn(&self, arg: Arg) -> _ {
    let ctx: Arc<Context> = Context::current();
    // ....
  }
}

The implementation will likely use task_local!. While tokio provides a really nice method scope that allows allocating the initially value upon entering the scope of the task local value, async-std doesn't provide such thing and thus may require a rather complicated way. This will not be a big problem if the each Context is essentially scoped within a client connection because we would just need to change the value once. However, if we were to add request specific info to the Context, then the task_local value will need to be initialized for each request execution.

[minghuaw]

Another Proposition

Another way would be to allow user define the service with the Context as the second parameter, which will not appear on the generated client stub (the macro takes care of this)

#[export_impl]
impl Service for State {
  #[export_method]
  pub async fn(&self, ctx: Arc<Context>, arg: Arg) -> _ {
    // ....
  }
}