fix: allow span links to be added to a SpanRef

[bantonsson]

Changes

This adds the ability to add a Link to a Span via a SpanRef. Since you can only access a SpanRef from the current Context, there was no way to add a new Link to that span.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.4%. Comparing base (1d9bd25) to head (650384f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2959     +/-   ##
=======================================
+ Coverage   81.3%   81.4%   +0.1%     
=======================================
  Files        126     126             
  Lines      24289   24305     +16     
=======================================
+ Hits       19763   19808     +45     
+ Misses      4526    4497     -29     

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[cijothomas]

LGTM.
Nit: add a changelog entry too as this is user facing addition!