Cache envelope headers

[SimonCropp]

• the sdk header can always be re-used. No need to spin up a new dictionary every time
• when there is no eventId then a static "default header" can be used.

[codecov-commenter]

Codecov Report

Merging #1242 (03be359) into main (71ff2ef) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1242      +/-   ##
==========================================
+ Coverage   80.59%   80.60%   +0.01%     
==========================================
  Files         212      212              
  Lines        6904     6908       +4     
  Branches     1587     1587              
==========================================
+ Hits         5564     5568       +4     
  Misses        809      809              
  Partials      531      531              

Impacted Files	Coverage Δ
src/Sentry/Envelopes/Envelope.cs	95.18% <100.00%> (+0.24%)	⬆️

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[SimonCropp]

@lucas-zimerman all changes applied