[Bug] S3 metadata timestamp truncates to nanosecond precision

[chachi]

Describe the bug

Truncation of NTP64 to rfc3339 nanosecond precision can cause storages to not find the correct timestamp and lose precision when loading data from S3.

Happens on conversion to string here: https://github.com/eclipse-zenoh/zenoh-backend-s3/blob/main/src/lib.rs#L259

To reproduce

Use the S3 storage and look at any timestamp. Its fraction portion will not be exactly identical to the stored portion in the S3 metadata.

System info

S3 backend with 0.11.0