Wire W3C propagator to HTTP & gRPC propagation

[marcotc]

This PR adds support for the newly introduced W3C Trace Context propagator.

This means that it is now possible to use the propagation style tracecontext, introduced in #2454, as a valid propagation style.

[ivoanjo]

This seems reasonable, although I haven't been following the overall change closely.

I grepped the codebase, and noticed that we usually also have the PROPAGATION_STYLE_... constants in settings.rb (+ settings_spec.rb) + all propagation styles in spec/datadog/tracing/distributed/propagation_spec.rb.

Do check if it makes sense to change those as well.

Otherwise, it LGTM 👍

[marcotc]

@ivoanjo thanks for bringing out these two points.
The PR as it stands is correct settings.rb & settings_spec.rb would only change if W3C propagation because a default value, which is not the case yet.

And even though propagation_spec.rb currently has all other propagation styles, the testing there is only relevant for Datadog vs non-Datadog propagation styles, so there's no reason to introduce W3C there despite B3 being that file.