Added JsonPbMarshalling interface

[petervandenbroek]

This allows for easy replacement by other marshallers.

See #235 for discussion on this.

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[johanbrandhorst]

Great! Thanks for this PR. One small suggestion.

[codecov-io]

Codecov Report

Merging #236 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #236   +/-   ##
=======================================
  Coverage   71.57%   71.57%           
=======================================
  Files          37       37           
  Lines        1351     1351           
=======================================
  Hits          967      967           
  Misses        335      335           
  Partials       49       49

Impacted Files	Coverage Δ
logging/logrus/payload_interceptors.go	84.12% <ø> (ø)	⬆️
logging/zap/payload_interceptors.go	84.61% <ø> (ø)	⬆️

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[petervandenbroek]

@googlebot I signed it!

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[petervandenbroek]

@googlebot I fixed it.

[petervandenbroek]

@googlebot I fixed it.

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[johanbrandhorst]

Thanks for your contribution!