Exposing pk in responses for resources

[daviddavis]

fixes #3633
https://pulp.plan.io/issues/3633

[dkliban]

Thank you!

[codecov]

Codecov Report

Merging #3481 into 3.0-dev will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           3.0-dev    #3481   +/-   ##
========================================
  Coverage    57.05%   57.05%           
========================================
  Files           59       59           
  Lines         2510     2510           
========================================
  Hits          1432     1432           
  Misses        1078     1078

Impacted Files	Coverage Δ
pulpcore/pulpcore/app/serializers/base.py	58.62% <100%> (ø)	⬆️

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