Extract envelope construction logic from Transport

[st0012]

Closes #1611

[codecov-commenter]

Codecov Report

Merging #1616 (2a45d15) into master (7aab67c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1616   +/-   ##
=======================================
  Coverage   98.49%   98.49%           
=======================================
  Files         131      132    +1     
  Lines        7322     7332   +10     
=======================================
+ Hits         7212     7222   +10     
  Misses        110      110           

Impacted Files	Coverage Δ
sentry-ruby/lib/sentry/envelope.rb	100.00% <100.00%> (ø)
sentry-ruby/lib/sentry/transport.rb	98.80% <100.00%> (-0.02%)	⬇️

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[st0012]

@sl0thentr0py does this look like what you'd expect?

[sl0thentr0py]

@st0012 yes that looks great!