feat: add w3 trace context support to cmd

[cewkrupa]

Which problem is this PR solving?

• Closes Generate 8-byte span IDs to conform with OpenTelemetry spec (instead of 16-byte currently) #171 (possibly)

Short description of the changes

Adds a --w3 boolean flag to cmd.

If this flag is set, instead of outputting a marshalled beeline trace propagation context into the HONEYCOMB_TRACE environment variable, cmdwill output a [W3 trace context](https://docs.honeycomb.io/troubleshoot/product-lifecycle/recommended-migrations/migrate-from-beelines/go/#interoperability-with-opentelemetry) into the environment variablesHONEYCOMB_W3_TRACEPARENTandHONEYCOMB_W3_TRACESTATE`.

How to verify that this has the expected result

Great question! I don't know how to test this

[cewkrupa]

totally open to a flag name that makes more sense

[cewkrupa]

for some reason TraceparentHeader is exported, but TracestateHeader isn't

[cewkrupa]

After discussing with @robbkidd -- even though this would generate W3C-compliant header values, these changes would not actually connect the build event traces to an OTel process on the other side of cmd. Closing this because it doesn't bring much value without also overhauling more of buildevent's internals to better support W3C/Otel.

[robbkidd]

The discussion with @cewkrupa sparked an idea. I have a hypothesis—a wholly untested hypothesis—that a judicious application of bash could go some way towards buildevents trace and span IDs being W3C compliant.

With inspiration from the not-officially-OpenTelemetry otel-bash project's generation of traceid and spanid:

• trace ID
  • generate a W3C trace ID at the very beginning of your CI workflow
  • persist that value consistently through all subsequent steps
  • make it available as an environment variable
  • use it in place of the CI provider's trace identifier in all buildevents invocations
• span ID
  • generate a W3C span id wherever STEP_SPAN_ID is being computed

With those two values consistently available, something running within a job step that needs a TRACEPARENT header can have one constructed from these two parts.