expose timestamp::ensure function

[milyin]

Describe the release item

This function was not exposed to zenoh-c because zenoh-c have only R/O access to sample. Consider if it can be added now

    /// Ensure that an associated Timestamp is present in this Sample.
    /// If not, a new one is created with the current system time and 0x00 as id.
    /// Get the timestamp of this sample (either existing one or newly created)
    // tags{rust.sample.ensure_timestamp, api.sample.timestamp.ensure}
    pub fn ensure_timestamp(&mut self) -> &Timestamp {
        if let Some(ref timestamp) = self.timestamp {
            timestamp
        } else {
            let timestamp = new_reception_timestamp();
            self.timestamp = Some(timestamp);
            self.timestamp.as_ref().unwrap()
        }
    }

[milyin]

first it have to be renamed: eclipse-zenoh/zenoh#810

[DenisBiryukov91]

Since we are moving to the concept of read-only Sample, this functionality is likely not needed to be exposed in zenoh-c.
Similar method is expected to be tagged as unstable and hidden in zenoh rust (see eclipse-zenoh/zenoh#822).

[milyin]

This function is removed in eclipse-zenoh/zenoh#806