fix(otel): Use vendored bagggage implementation in propagator

[tonyo]

After #568, we can finally fix the issue in the propagator that resulted in dropping baggage.
Now we only use our vendored baggage implementation with the fixes that hopefully make our implementation consistent with baggage implementations in other Sentry SDKs.

[codecov]

Codecov Report

Base: 79.13% // Head: 79.14% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (a835f6f) compared to base (3964ece).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #573      +/-   ##
==========================================
+ Coverage   79.13%   79.14%   +0.01%     
==========================================
  Files          38       38              
  Lines        3853     3856       +3     
==========================================
+ Hits         3049     3052       +3     
  Misses        703      703              
  Partials      101      101              

Impacted Files	Coverage Δ
otel/propagator.go	92.00% <100.00%> (+0.33%)	⬆️

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