[parametric] Enable the W3C Phase 3 test for cpp

[zacharycmontoya]

Motivation

Ensures that our handling of the W3C Trace Context headers is complete and provides the best distributed tracing experience.

Changes

Enables the following test cases for cpp:

• tests/parametric/test_headers_tracecontext.py::Test_Headers_Tracecontext::test_tracestate_w3c_p_extract_datadog_w3c

Depends on functionality introduced in DataDog/dd-trace-cpp#142

Workflow

1. ⚠️ Create your PR as draft ⚠️
2. Work on you PR until the CI passes (if something not related to your task is failing, you can ignore it)
3. Mark it as ready for review
   • Test logic is modified? -> Get a review from RFC owner. We're working on refining the codeowners file quickly.
   • Framework is modified, or non obvious usage of it -> get a review from R&P team

🚀 Once your PR is reviewed, you can merge it!

🛟 #apm-shared-testing 🛟

Reviewer checklist

• If PR title starts with [<language>], double-check that only <language> is impacted by the change
• No system-tests internal is modified. Otherwise, I have the approval from R&P team
• CI is green, or failing jobs are not related to this change (and you are 100% sure about this statement)
• A docker base image is modified?
  • the relevant build-XXX-image label is present
• A scenario is added (or removed)?
  • Get a review from R&P team

[zacharycmontoya]

Note: This is expected to fail until the related PR gets merged. Until then, I will keep this in draft

[zacharycmontoya]

Note: The cpp parametric tests are passing, which indicates this update is successful

[mabdinur]

This PR is blocked by: DataDog/dd-trace-cpp#170.

The cpp parametric app does not support the new trace/span/extract_headers endpoint. Currently all parametric tests that generate spans from http headers are skipped.

cc: @dmehala