Add default Retry-After value

[mattjohnsonpint]

Completes #1443

Refactored the logic for better readability as well.

[codecov-commenter]

Codecov Report

Merging #1537 (91dba30) into main (6268305) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1537      +/-   ##
==========================================
+ Coverage   84.30%   84.33%   +0.03%     
==========================================
  Files         219      219              
  Lines        7417     7419       +2     
  Branches     1418     1418              
==========================================
+ Hits         6253     6257       +4     
  Misses        736      736              
+ Partials      428      426       -2     

Impacted Files	Coverage Δ
src/Sentry/Internal/Http/RetryAfterHandler.cs	100.00% <100.00%> (+4.16%)	⬆️
src/Sentry/Exceptions/SentryStackFrame.cs	78.02% <0.00%> (+1.09%)	⬆️

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