Exposes ContentFilter for plugins

[vdusek]

re #3783
https://pulp.plan.io/issues/3783

[pep8speaks]

Hello @vdusek! Thanks for submitting the PR.

• There are no PEP8 issues in the file plugin/pulpcore/plugin/viewsets.py !

• There are no PEP8 issues in the file pulpcore/pulpcore/app/viewsets/__init__.py !

[pulpbot]

Can one of the admins verify this patch?

[codecov]

Codecov Report

Merging #3674 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3674   +/-   ##
=======================================
  Coverage   55.22%   55.22%           
=======================================
  Files          64       64           
  Lines        2754     2754           
=======================================
  Hits         1521     1521           
  Misses       1233     1233

Impacted Files	Coverage Δ
pulpcore/pulpcore/app/viewsets/__init__.py	100% <100%> (ø)	⬆️

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[dralley]

Please sort imports alphabetically. Otherwise, looks good.

[vdusek]

@dralley Fixed, thank you.