airframe-rpc: Generate Open API schema from RPC interfaces

[xerial]

• Define OpenAPI model classes
  • Support UnionType codec for using A or B in OpenAPI model classes.
• Create OpenAPI model class from RPC interfaces
• Create OpenAPI model class from Endpoint interfaces
• Convert JSON to Yaml
• Add sbt-airframe plugin command (airframeHttpOpenAPIXXX)
• Usage note

[codecov]

Codecov Report

Merging #1178 into master will increase coverage by 0.31%.
The diff coverage is 87.15%.

@@            Coverage Diff             @@
##           master    #1178      +/-   ##
==========================================
+ Coverage   82.60%   82.92%   +0.31%     
==========================================
  Files         279      283       +4     
  Lines       10707    10941     +234     
  Branches      698      727      +29     
==========================================
+ Hits         8845     9073     +228     
- Misses       1862     1868       +6     

Impacted Files	Coverage Δ
...la/wvlet/airframe/http/codegen/RouteAnalyzer.scala	95.45% <ø> (ø)
...vlet/airframe/http/codegen/HttpCodeGenerator.scala	19.14% <21.05%> (ø)
...airframe/http/finagle/FinagleResponseHandler.scala	85.71% <57.14%> (-2.82%)	⬇️
...main/scala/wvlet/airframe/json/JSONTraverser.scala	58.33% <57.14%> (+58.33%)	⬆️
...c/main/scala/wvlet/airframe/codec/UnionCodec.scala	66.66% <66.66%> (ø)
...in/scala/wvlet/airframe/http/openapi/OpenAPI.scala	86.66% <86.66%> (ø)
...wvlet/airframe/http/openapi/OpenAPIGenerator.scala	95.38% <95.38%> (ø)
...cala/wvlet/airframe/codec/MessageCodecFinder.scala	87.80% <100.00%> (+0.62%)	⬆️
...vm/src/main/scala/wvlet/airframe/http/Router.scala	90.76% <100.00%> (+2.70%)	⬆️
...ame/http/router/HttpEndpointExecutionContext.scala	95.23% <100.00%> (ø)
... and 14 more

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[xerial]

@shimamoto @takezoe This becomes a big PR, but the basic implementation for generating Open API schema from airframe-http/rpc endpoints is done. Please have a look.

[xerial]

@wvlet/airframe-dev I think reviewing this PR is hard, so let me merge this PR first as a beta-feature because this PR should have no production behavior change.