fix: Dynamically lookup schema when simply a module

[oceanlewis]

Changes the Cast logic to attempt to lookup the schema when given a module as a :schema.

Addresses these issues:

• https://github.com/open-api-spex/open_api_spex/issues/627
• https://github.com/open-api-spex/open_api_spex/issues/626

[mbuhot]

Thanks for the PR @oceanlewis. I'm closing this one since OpenAPISpex is designed to have Schema modules resolved to %References{} immediately after constructing the %OpenApi{}, as per the example in the README:

  @impl OpenApi
  def spec do
    %OpenApi{
      servers: [
        # Populate the Server info from a phoenix endpoint
        Server.from_endpoint(Endpoint)
      ],
      info: %Info{
        title: "My App",
        version: "1.0"
      },
      # Populate the paths from a phoenix router
      paths: Paths.from_router(Router)
    }
    |> OpenApiSpex.resolve_schema_modules() # Discover request/response schemas from path specs
  end