support to add list of required fields to objects generated in swagger.json

[srikanth6k]

In .proto file for each message we can add openapiv2_schema.json_schema option to generate some metadata about the message in swagger.json. As of now only title and description are supported. This change enables them to generate a list of required fields.

[codecov-io]

Codecov Report

Merging #716 into master will decrease coverage by 0.05%.
The diff coverage is 25%.

@@            Coverage Diff             @@
##           master     #716      +/-   ##
==========================================
- Coverage   56.43%   56.38%   -0.06%     
==========================================
  Files          30       30              
  Lines        3053     3056       +3     
==========================================
  Hits         1723     1723              
- Misses       1159     1162       +3     
  Partials      171      171

Impacted Files	Coverage Δ
protoc-gen-swagger/genswagger/types.go	19.04% <ø> (ø)	⬆️
protoc-gen-swagger/genswagger/template.go	38.04% <25%> (-0.15%)	⬇️

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[johanbrandhorst]

Looks to me like support for this was merged with #687