feat(scopes): Add sentry_scope_clear()

[limbonaut]

Adds sentry_scope_clear, which resets the given scope data (user, tags, breadcrumbs, attributes, and so on) to a fresh state while keeping its trace, so a reusable scope can be cleared and used again.

• Refs Make scopes more aligned to Sentry specs to ease support in the downstream SDKs #1838

[github-actions]

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📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

Generated by 🚫 dangerJS against 91efc56

[limbonaut]

I aligned this implementation with Cocoa and Android. I didn't find much details on scope.clear() in the specs, so borrowed this part from other SDKs.

spec: Reset scope to defaults, keeping event processors.

It makes sense that the scope.clear() doesn't drop trace defaults.

[limbonaut]

In the current shape, this is close to no-op, because user-created scopes carry no default trace, and it's picked up from the global scope instead. But this may change if we introduce propagation context mutators.

[codecov]

Codecov Report

❌ Patch coverage is 89.47368% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.60%. Comparing base (947def6) to head (91efc56).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1881      +/-   ##
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+ Coverage   75.56%   75.60%   +0.04%     
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  Files          90       90              
  Lines       21169    21188      +19     
  Branches     3763     3764       +1     
==========================================
+ Hits        15996    16020      +24     
+ Misses       4346     4333      -13     
- Partials      827      835       +8     

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