feat(tvix/tracing): http propagation for axum

It introduces a new accept_trace function for axum0.7 which can be used
to accept a header trace from a received request. This function can be
used for tonic 0.12 once that version is released, and the specific
`accept_trace` function within `tvix_tracing::propagate::tonic` can then
be removed.

This also integrates http propagation into the nar_bridge crate.

Change-Id: I46dcc797d494bb3977c2633753e7060d88d29129
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11925
Reviewed-by: Brian Olsen <me@griff.name>
Tested-by: BuildkiteCI
Reviewed-by: Simon Hauser <simon.hauser@helsinki-systems.de>
Reviewed-by: flokli <flokli@flokli.de>

tvix/tracing/Cargo.toml

@@ -24,6 +24,8 @@ http  = { version = "0.2.11", optional = true }
 
 reqwest-tracing = { version = "0.4.8", default-features = false, optional = true }
 
+axum = { version = "0.7.5", optional = true }
+
 [features]
 default = []
 otlp = [
@@ -44,6 +46,9 @@ tonic = [
 reqwest = [
   "dep:reqwest-tracing",
 ]
+axum = [
+  "dep:axum",
+]
 
 [lints]
 workspace = true