Add --quiet option for spec generation

[Cowa]

This PR adds a --quiet option on the spec generation task.
It allows to disable the mix shell printing when creating folders and files.

Useful to avoid pollution if you run the task in tests, for example:

[zorbash]

@Cowa Makes sense to be able to silence the output of the mix tasks but I'm curious to know what is the purpose of the your tests running these tasks. Can you please explain what you're testing?

[Cowa]

@zorbash We use orval on the front-end, it uses the generated json spec to create a TypeScript client.

The json spec is committed and the test ensures that if the API has changed then the json spec was re-generated.
This way the CI fails if it's not up-to-date.

The test just run the task and compares the generated spec against the committed one.

test "latest API specs should have been generated", %{tmp_path: tmp_path} do
  Mix.Tasks.Openapi.Spec.Json.run([
    "--spec",
    "EmployerBrandingWeb.AnalyticsApiSpec",
    "--start-app=false",
    "--pretty=true",
    tmp_path
  ])

  latest_specs = File.read!(tmp_path)
  local_specs = File.read!("doc_src/api_specs/analytics.json")

  assert latest_specs == local_specs,
          "Run `mix openapi.gen` and commit changes - OpenApi specs have been modified but not re-generated"
end