Promote Span addLink to stable API

[jack-berg]

AddLink was stabilized in the spec here: open-telemetry/opentelemetry-specification#3887

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.02%. Comparing base (7926c45) to head (441a4ce).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6317      +/-   ##
============================================
- Coverage     91.04%   91.02%   -0.02%     
+ Complexity     5726     5725       -1     
============================================
  Files           625      624       -1     
  Lines         16744    16744              
  Branches       1713     1713              
============================================
- Hits          15244    15242       -2     
  Misses         1006     1006              
- Partials        494      496       +2     

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[jkwatson]

public API change approved (didn't review the implementation much)