Add custom headers to HTTP traces

[bgins]

We recently added otelmux to auto-instrument our Solver server with HTTP traces (#388). The auto-instrumentation collects common HTTP headers, but it would be nice to also see the x- headers we add for address, signature, and version.

Let's update our implementation to extract these headers. On a quick look, we will want to write a custom propagator that extracts the headers and adds them on the trace.