chore: Refactor SentryAttributes into a separate class

[jamescrosswell]

Before closing #4912 I figured we could extract some of the refactorings that might be useful in other PRs dealing with Attributes. Basically there's some functionality that is only accessible in the SentryMetrics class at the moment, but which will be needed by any other classes that also have Attributes.

#skip-changelog

[github-actions]

Semver Impact of This PR

⚪ None (no version bump detected)

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[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.88%. Comparing base (f201b65) to head (8ec35ea).

Files with missing lines	Patch %	Lines
src/Sentry/Protocol/SentryAttributes.cs	68.75%	13 Missing and 2 partials ⚠️
src/Sentry/SentryMetric.Factory.cs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4936      +/-   ##
==========================================
- Coverage   73.90%   73.88%   -0.03%     
==========================================
  Files         496      497       +1     
  Lines       17951    17960       +9     
  Branches     3516     3516              
==========================================
+ Hits        13266    13269       +3     
- Misses       3824     3830       +6     
  Partials      861      861              

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