feat: Add the OtlpTraceLayer (#33)

This layer collects and sends traces to the OTLP collector.
Trace and Span IDs are now set on Span extensions instead of reusing the
tracing generated IDs.

fix: oci-spec update outputs pretty manifests

With oci-spec 0.6.6 [#166](youki-dev/oci-spec-rs#166)
the `to_string()` method started to return the
blanket impl of `ToString` due to the `Display` Trait being added.

I'm not entirely sure why I got the display trait instead of the
`to_string()` implemented on `ImageManifest`, in the rust playground I'm
unable to reproduce it. Instead I now call serde_json directly so I
don't have to rely on whatever oci-spec implements as the serialization.