Connect Sidekiq's transaction with its parent when possible

[st0012]

If there's a parent transaction on the client side, its sentry_trace should be passed and picked by the server side middleware, so the 2 transactions can connect.

Closes #1586

[codecov-commenter]

Codecov Report

Merging #1590 (e588ef2) into master (5c17674) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1590   +/-   ##
=======================================
  Coverage   98.41%   98.42%           
=======================================
  Files         129      129           
  Lines        7192     7218   +26     
=======================================
+ Hits         7078     7104   +26     
  Misses        114      114           

Impacted Files	Coverage Δ
...iq/lib/sentry/sidekiq/sentry_context_middleware.rb	97.29% <100.00%> (+0.52%)	⬆️
...c/sentry/sidekiq/sentry_context_middleware_spec.rb	100.00% <100.00%> (ø)

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