feat(tvix/tracing): http trace propagation

Introduces a helper function within tvix-tracing that returns a reqwest
tracing middleware that will ingest the traceparent if otlp is enabled.

It is feature flagged in tvix-tracing so not every consumer of that
library automatically has reqwest in its dependencies.

Tested using netcat to verify that the `traceparent` header is there if
otlp is enabled and missing if otlp feature is disabled.

Change-Id: I5abccae777b725f5ff7382e3686165383c477a39
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11886
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/docs/src/TODO.md

@@ -224,7 +224,6 @@ logs etc, but this is something requiring a lot of designing.
  - Maybe drop `--log-level` entirely, and only use `RUST_LOG` env exclusively?
    `debug`,`trace` level across all crates is a bit useless, and `RUST_LOG` can
    be much more granular…
- - Trace propagation for HTTP clients too, using
-   https://www.w3.org/TR/trace-context/ or https://www.w3.org/TR/baggage/,
-   whichever makes more sense.
-   Candidates: nix+http(s) protocol, object_store crates.
+ - Trace propagation for object_store once they support a way to register a
+   middleware, so we can use that to register a tracing middleware.
+   https://github.com/apache/arrow-rs/issues/5990