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Remove tracing dependency from http-lib and add baggage (#6065)

Removes dependency on `tracing` from `http-lib` and add baggage.

---

The introduction of it as a dependency was to support this style of
code:

```ocaml
let@ req = Http.Request.with_tracing ~name req in
```

However, HTTP headers are just a carrier in the context of tracing. The
library ought to be as inert as possible; providing enough to endow
requests with tracing information, but not calling into the tracing
library to export child spans (as is done above).

To this end, the notion of a
[propagator](https://opentelemetry.io/docs/specs/otel/context/api-propagators/)
is introduced that provides an interface for doing the above, but
generalised to arbitrary carriers. If you can describe how to
inject/extract a "trace context" into/from a carrier, you can
conceivably propagate tracing across it.

---

In future, the idea will be to:
- This trace context can be inherited by spans, so it can reach the
exporter. We can probably use the `SpanContext.t` record to do this. If
this information can reach the exporter, we can choose to decorate spans
with tags derived from the trace context (such as baggage) during
exportation.
- At incoming service boundaries, instead of pushing down a `Span.t
option` derived from an encoded `traceparent` in the carrier, we will
push down `TraceContext` (which is more general than the encoded
`SpanContext` carried by the optional `Span.t` that's currently pushed
down). This is so we can more easily handle the case where baggage is
provided but a traceparent isn't: in which case, a span is still created
if tracing is enabled, but with no parent to inherit contextual metadata
from (a previous - abandoned - PR introduced baggage in a way that only
worked when a traceparent was provided).

---

Opening this as a draft for now. Would appreciate any commentary. There
appears to be duplication of `Helper` but this is largely because XAPI's
`dune` defines separate lib... (continued)

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