example: Add example of user authencate l2 switch like an AP on cafe

examples/cafe/.formatter.exs

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+# Used by "mix format"
+[
+  inputs: ["mix.exs", "config/*.exs"],
+  subdirectories: ["apps/*"]
+]

examples/cafe/.gitignore

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+# The directory Mix will write compiled artifacts to.
+/_build/
+
+# If you run "mix test --cover", coverage assets end up here.
+/cover/
+
+# The directory Mix downloads your dependencies sources to.
+/deps/
+
+# Where third-party dependencies like ExDoc output generated docs.
+/doc/
+
+# Ignore .fetch files in case you like to edit your project deps locally.
+/.fetch
+
+# If the VM crashes, it generates a dump, let's ignore it too.
+erl_crash.dump
+
+# Also ignore archive artifacts (built via "mix archive.build").
+*.ez
+

examples/cafe/README.md

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+# Cafe
+
+**TODO: Add description**
+

examples/cafe/config/config.exs

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+# This file is responsible for configuring your application
+# and its dependencies with the aid of the Mix.Config module.
+use Mix.Config
+
+# By default, the umbrella project as well as each child
+# application will require this configuration file, as
+# configuration and dependencies are shared in an umbrella
+# project. While one could configure all applications here,
+# we prefer to keep the configuration of each individual
+# child application in their own app, but all other
+# dependencies, regardless if they belong to one or multiple
+# apps, should be configured in the umbrella to avoid confusion.
+import_config "../apps/*/config/config.exs"
+
+# Sample configuration (overrides the imported configuration above):
+#
+#     config :logger, :console,
+#       level: :info,
+#       format: "$date $time [$level] $metadata$message\n",
+#       metadata: [:user_id]

examples/cafe/mix.exs

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+defmodule Cafe.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      apps_path: "apps",
+      start_permanent: Mix.env() == :prod,
+      deps: deps()
+    ]
+  end
+
+  # Dependencies listed here are available only for this
+  # project and cannot be accessed from applications inside
+  # the apps folder.
+  #
+  # Run "mix help deps" for examples and options.
+  defp deps do
+    []
+  end
+end