[WIP] pkg/generator: add cmd/main.go generator

[fanminshi]

add github.com/dave/jennifer/jen dependency.
use Jennifer code generator to auto generate *.go files.
generate cmd/main.go as part of operator application generation.

[fanminshi]

this depends on #21 because it uses some constants defined in that pr.

[hongchaodeng]

I would suggest to use go library text/template to generate template files instead of using jennifer.

First of all, the library itself is very confusing. Looking at the code:

	// func main() {..}
	file.Func().Id("main").Params().Block(
		// namespace := "default"
		Id("namespace").Op(":=").Lit("default"),
		// sdk.Watch(...)
		Qual(sdkImport, "Watch").Call(
			Qual(apiImport, servicePlural),
			Id("namespace"),
			Qual(apiImport, service),
		),
		// sdk.Handle(...)
		Qual(stubImport, "Handle").Call(
			Op("&").Qual(stubImport, "Handler{}"),
		),
		// sdk.Run(...)
		Qual(sdkImport, "Run").Call(
			Qual("context", "TODO").Call(),
		),
	)

Every line of code needs explanation in order to understand it. The template should just describe it itself.

Second, the jennifer library hasn't seen major activities for more than 6 months: https://github.com/dave/jennifer/graphs/contributors.

[fanminshi]

@hongchaodeng I can give text/template a try. what's nice about jennifer is that it handles imports nicely and also it is strict.

[hongchaodeng]

I understand that jennifer has put some syntax meaning into the library. It's a good direction. But it is too early and immature a project.

[fanminshi]

@hongchaodeng let me explore other code generation tools.

[fanminshi]

close this in favor of #29