[design] Generate http server & client based on code/spec

[at15]

This issue is track the trail implementation and references. It is originated from the discussion in https://github.com/dyweb/weekly/issues/91

The main idea is to generate code to avoid manually write json encode/decode and stub func GetXXX so that writing REST API can feel like writing RPC.

There are following approaches

• write spec in YAML like OpenAPI and generate client and server stub
  • this limit how we can struct our server code and we need to learn how to plug into the java based generator, add more template I guess @at15
• write server interface in go code, generate spec (i.e. OpenAPI, Protobuf) from go code, and generate client using existing tools
• come up with a new spec like OpenAPI or Protobuf

And some extra rules

• all generation should happen at compile time, no runtime detect and interface{}

@at15 prefer the middle one (currently)

Reference

• https://caicloud.github.io/nirvana/zh-hans/guides/client.html
• https://github.com/goadesign/goa write the spec in go as a DSL

Newly added

• https://github.com/swaggo/swag generate swagger based on comment in go