Add Accept header information to the Input struct

[czechboy0]

This is a follow-up task to #6.

• Synthesizing an Input parameter that's a type-safe variant of the "accept" header
• The synthesized accept enum should only contain supported content types that we actually generate Output cases for

This way, the server implementer, conforming their type to APIProtocol, will have a clear signal from the user about which content type they want.

Things to keep in mind:

• Client might not care
• Client might specify */*
• Client might specify application/*
• Client might specify multiple, and use priorities (the q parameter)

The synthesized enum might not be concrete types, but some representation like:

enum Accept {
  case any
  case anySubtype(String)

  enum SupportedContentTypes {
    case json
    case yaml
  }

  case concrete(SupportedContentTypes)

  // The order here is significant, sorted from most to least desired, according to the rules in https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept
  case multiple([SupportedContentTypes]
}

This will require a proposal, as there are many options and it needs to be carefully considered.

[czechboy0]

Was the subject of SOAR-0003: #201.

[czechboy0]

Will land in 0.1.11 behind the feature flag multipleContentTypes.