feat(cmd/oapi-codegen): add flag to generate subset of code

[goller]

Hey @deepmap-marcinr , I wanted to get your feedback on how I did flags first before having the Generate code use them.

What do you think?

This adds a -generate option. It accepts, all, server, client, and types.

By default it uses all.

This is the output when an unknown option is passed:

./oapi-codegen  -generate howdy
invalid value "howdy" for flag -generate: must be "all", "server", "client", or "types"
Usage of ./oapi-codegen:
  -generate value
        The code to generate; valid options: all (default), server, client, types
  -package string
        The package name for generated code

[deepmap-marcinr]

Does server or client imply types? For example, if I want only the server code, I would need both the types and the server, for the client, I would need the types and the client code.

Maybe -generate can take a list, like --generate=types,client? This way, you can default it to types,server,client instead of "all"?

[goller]

@deepmap-marcinr Good call... I like the list. I'll make that change.

Are you ok with client/server and types being split out? Should server and client imply types?

[deepmap-marcinr]

Hmm, that's an interesting question. Certainly, the Client and Server interfaces should be split out, and any helper functions. So, if there's a RequestParams object for an operation, it will be needed in both the client and server. You know, as a first attempt, just to make the code simpler, just generate all types together, independent of the client/server functions. They might be useful in code that operates on those types, so better to err on the side of too much than too little.

[goller]

@deepmap-marcinr right on, ok

[goller]

@deepmap-marcinr agreed... current code gets my go spidey-sense tingling... too complicated. I'll switch it to a straight up split for sure.

[goller]

@deepmap-marcinr I've updated the CLI handling and did a small refactor of the server/client generation to use the options.

The CLI parameter is now a comma delimited set of choices.

Here is the error message with an unknown option:

./oapi-codegen -generate howdy swagger.yml
unknown generate option howdy
  -generate string
        Comma-separated list of code to generate; valid options: "client", "server"  (default client,server) (default "client,server")
  -package string
        The package name for generated code

[deepmap-marcinr]

Thank you.