feat: Add span links to the Datadog API

[mabdinur]

What does this PR do?

Introduces an API for generating and serializing span links using the Datadog API. Support for OTEL span links will be introduced in a future PR.

Inspiration: https://github.com/open-telemetry/opentelemetry-ruby/blob/b1d6d050c613ad81f51dbe6db498815deeb110d4/api/lib/opentelemetry/trace/link.rb#L13

Motivation:

Span Links provides a new mechanism to correlate spans.

Additional Notes:

• This change requires datadog-agent v7.41. datadog-agent<7.41 does not support deserializing span links.
• This is not a breaking change to the v0.4 trace API. Older versions of the agent will ignore the span_link field in trace payloads.

How to test the change?

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.11%. Comparing base (22f8d40) to head (c6431dd).
Report is 3 commits behind head on 2.0.

Additional details and impacted files

@@           Coverage Diff            @@
##              2.0    #3546    +/-   ##
========================================
  Coverage   98.11%   98.11%            
========================================
  Files        1236     1238     +2     
  Lines       72179    72303   +124     
  Branches     3396     3408    +12     
========================================
+ Hits        70815    70940   +125     
+ Misses       1364     1363     -1     

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