Encapsulate extension helpers

[st0012]

As mentioned in #1708, some extensions from the SDK expose internal methods publicly. From what I can tell this includes sentry-rails's ActiveJob integration and sentry-resque. This fixes the issue.

[codecov-commenter]

Codecov Report

Merging #1725 (90e9f1f) into master (623d046) will increase coverage by 0.01%.
The diff coverage is 98.36%.

@@            Coverage Diff             @@
##           master    #1725      +/-   ##
==========================================
+ Coverage   98.40%   98.41%   +0.01%     
==========================================
  Files         141      141              
  Lines        8012     8019       +7     
==========================================
+ Hits         7884     7892       +8     
+ Misses        128      127       -1     

Impacted Files	Coverage Δ
sentry-resque/lib/sentry/resque.rb	97.14% <96.66%> (-2.86%)	⬇️
sentry-rails/lib/sentry/rails/active_job.rb	100.00% <100.00%> (ø)
sentry-ruby/lib/sentry/background_worker.rb	100.00% <0.00%> (+5.40%)	⬆️

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[lewispb]

LGTM!