[WIP] Enable Span Links for Node

[sabrenner]

Motivation

Enabling span links for the node tracer.

Changes

Adds additional logic for the Node.js server to handle links as part of the startSpan request and adds the appropriate endpoint (/trace/span/add_link) for adding a link to a span.

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

:rocket: Once your PR is reviewed, you can merge it!

🛟 #apm-shared-testing 🛟

Reviewer checklist

• [ ] Relevant label (run-parametric-scenario, run-profiling-scenario...) are presents
• [ ] 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
  • [ ] To R&P team: locally build and push the image to hub.docker.com
• [ ] A scenario is added (or removed)?
  • [ ] Get a review from R&P team
  • [ ] Once merged, add (or remove) it in system-test-dasboard nightly