move SemanticConventions from shared to API project

[SimonCropp]

Fixes #
Design discussion issue #

Changes

and leverage internal visible for OpenTelemetry.Exporter.Zipkin.
also reduces the size of the OpenTelemetry.Exporter.Zipkin combo (dll,pdb,xml) by 28k

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.39%. Comparing base (89ea5c6) to head (82b9879).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6094   +/-   ##
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  Coverage   86.39%   86.39%           
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  Files         257      257           
  Lines       11640    11640           
=======================================
  Hits        10056    10056           
  Misses       1584     1584           

[TimothyMothra]

overall LGTM, but need to discuss with all maintainers if this is the direction we want to go.

[TimothyMothra]

This change needs to be discussed by maintainers.
We've historically been hesitant to make internals visible to other libraries.
I understand there's precedence for other Exporters (OTLP and Console and InMemory), but I don't know the longterm plans for the Zipkin Exporter.

[TimothyMothra]

This was discussed in the 2/4 SIG meeting.

The decision is that InternalsVisibleTo is the wrong direction to go. The fact that internals are visible to other libraries is considered technical debt and the goal is to eventually remove these.