Generate a single swagger definition on demand

[achew22]

Combines the efforts of @cy-zheng in #643 with the efforts of @zxy198717 in #591.

Fixes: #643, #591, #99

[googlebot]

So there's good news and bad news.

👍 The good news is that everyone that needs to sign a CLA (the pull request submitter and all commit authors) have done so. Everything is all good there.

😕 The bad news is that it appears that one or more commits were authored or co-authored by someone other than the pull request submitter. We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that here in the pull request.

Note to project maintainer: This is a terminal state, meaning the cla/google commit status will not change from this state. It's up to you to confirm consent of the commit author(s) and merge this pull request when appropriate.

[achew22]

I manually edited both changes to leave authorship with their respective author, rebased and uploaded. This can now fix a long standing issue. Thanks so much everyone!

[hexfusion]

@achew22 any idea when this might be released? Would it make 1.4.x?

[codecov-io]

Codecov Report

Merging #658 into master will decrease coverage by 1.15%.
The diff coverage is 6.84%.

@@            Coverage Diff             @@
##           master     #658      +/-   ##
==========================================
- Coverage   58.94%   57.78%   -1.16%     
==========================================
  Files          30       30              
  Lines        2857     2914      +57     
==========================================
  Hits         1684     1684              
- Misses       1010     1066      +56     
- Partials      163      164       +1

Impacted Files	Coverage Δ
protoc-gen-swagger/genswagger/generator.go	0% <0%> (ø)	⬆️
protoc-gen-grpc-gateway/descriptor/registry.go	71.03% <0%> (-3.25%)	⬇️
protoc-gen-swagger/genswagger/template.go	42.1% <100%> (-0.25%)	⬇️
protoc-gen-swagger/main.go	27.38% <42.85%> (+1.4%)	⬆️

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[achew22]

@hexfusion ask and you shall receive https://github.com/grpc-ecosystem/grpc-gateway/releases/tag/v1.4.1

[hexfusion]

Perfect thanks a lot!