Add custom UnmarshalJSON methods for modified conduit api responses

[schleppy]

Add custom UnmarshalJSON methods for DifferentialRevision and DifferentialQueryRequest to handle change in reviewers type from []string to map[string]string in conduit api. Each value in the map is the same as the key, so we just need to read the keys

[codecov]

Codecov Report

Merging #70 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #70   +/-   ##
=======================================
  Coverage   20.32%   20.32%           
=======================================
  Files          25       25           
  Lines         925      925           
=======================================
  Hits          188      188           
  Misses        735      735           
  Partials        2        2

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