Add support for Protobuf format response

[salamer]

Gin now have the protobufBinding function to check the request format, but didn't have a protobuf response function like c.YAML().
In our company ByteDance, the largest internet company using golang in China, we use gin to transfer Protobuf instead of Json, we have to write some internal library to make some wrappers to achieve that, and the code is not elegant. So we really want such a feature.

[codecov]

Codecov Report

Merging #1475 into master will decrease coverage by 0.57%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1475      +/-   ##
==========================================
- Coverage   98.88%   98.31%   -0.58%     
==========================================
  Files          36       37       +1     
  Lines        1887     1898      +11     
==========================================
  Hits         1866     1866              
- Misses         16       27      +11     
  Partials        5        5

Impacted Files	Coverage Δ
render/render.go	100% <ø> (ø)	⬆️
context.go	98.97% <0%> (-0.52%)	⬇️
render/protobuf.go	0% <0%> (ø)

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[appleboy]

duplicated of #1477