allow access to the config of the created session

[milyin]

Describe the release item

It's possible to access config of the working session in zenoh. The zenoh-c can't do it

    /// ### Modify current zenoh configuration
    /// ```
    /// # async_std::task::block_on(async {
    /// use zenoh::prelude::r#async::*;
    ///
    /// let session = zenoh::open(config::peer()).res().await.unwrap();
    /// let _ = session.config().insert_json5("connect/endpoints", r#"["tcp/127.0.0.1/7447"]"#);
    /// # })
    /// ```
    // tags{rust.session.config, api.session.config{set,get}}
    pub fn config(&self) -> &Notifier<Config> {
        self.runtime.config()
    }