Add fallback to Scope Stack from AspNet.

[lucas-zimerman]

This will stop the null reference exceptions when using the ASP.NET SDK during any Scope interaction that happens before a HttpContext is created.

Close #1157.

[codecov-commenter]

Codecov Report

Merging #1180 (2b24862) into main (5a305f8) will decrease coverage by 0.00%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##             main    #1180      +/-   ##
==========================================
- Coverage   80.49%   80.49%   -0.01%     
==========================================
  Files         212      212              
  Lines        6846     6851       +5     
  Branches     1563     1565       +2     
==========================================
+ Hits         5511     5515       +4     
  Misses        823      823              
- Partials      512      513       +1     

Impacted Files	Coverage Δ
....AspNet/Internal/HttpContextScopeStackContainer.cs	42.85% <42.85%> (+42.85%)	⬆️
src/Sentry/GlobalSessionManager.cs	69.86% <0.00%> (+0.43%)	⬆️

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