Adding support for more well known types in descriptor

[mayankcpdixit]

This change is to support more wellknown types in "protoc-gen-grpc-gateway". It solves the following issue:
Ref: #808

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[codecov-io]

Codecov Report

Merging #809 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #809   +/-   ##
======================================
  Coverage    53.3%   53.3%           
======================================
  Files          30      30           
  Lines        3371    3371           
======================================
  Hits         1797    1797           
  Misses       1399    1399           
  Partials      175     175

Impacted Files	Coverage Δ
protoc-gen-grpc-gateway/descriptor/types.go	41.08% <ø> (ø)	⬆️

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[mayankcpdixit]

I signed it!

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