add cas auth provider strategy

lib/cadet/auth/providers/cas.ex

@@ -0,0 +1,43 @@
+defmodule Cadet.Auth.Providers.CAS do
+  @moduledoc """
+  Provides identity using CAS Protocol.
+  https://apereo.github.io/cas/6.5.x/protocol/CAS-Protocol.html
+  """
+
+  alias Cadet.Auth.Provider
+
+  @behaviour Provider
+
+  @type config :: %{service_validate_endpoint: String.t(), modules: %{}}
+
+  @spec authorise(config(), Provider.code(), Provider.client_id(), Provider.redirect_uri()) ::
+          {:ok, %{token: Provider.token(), username: String.t()}}
+          | {:error, Provider.error(), String.t()}
+  def authorise(config, code, _client_id, redirect_uri) do
+    params = %{
+      ticket: code,
+      service: redirect_uri
+    }
+
+    with {:validate, {:ok, %{body: body, status_code: 200}}} <-
+           {:validate, HTTPoison.get(config.service_validate_endpoint, [], params: params)},
+         {:validation_response, data} <- {:validation_response, Jason.decode!(body)},
+         {:extract_username, %{"name" => username}} <- {:extract_username, data} do
+          IO.inspect(data)
+      {:ok, %{token: data, username: username}}
+    else
+      {:validate, {:ok, %{body: body, status_code: status}}} ->
+        {:error, :upstream, "Status code #{status} from CAS: #{body}"}
+    end
+  end
+
+  @spec get_name(config(), Provider.token()) ::
+          {:ok, String.t()} | {:error, Provider.error(), String.t()}
+  def get_name(_config, token) do
+    %{"name" => name} = token
+    {:ok, name}
+  rescue
+    _ ->
+      {:error, :invalid_credentials, "Failed to retrieve user's name"}
+  end
+end