release: add Rack application skeleton of webhook auto signer

GitHub: GH-43

In this PR, we set up a rack application skeleton of webhook auto
signer. At the following PRs, we will implement the logic.

Author: otegami

ansible/files/home/signer/webhook/.gitignore

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+/Gemfile.lock

ansible/files/home/signer/webhook/Gemfile

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+source "https://rubygems.org"
+
+gem "rack"

ansible/files/home/signer/webhook/config.ru

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+require "pathname"
+
+base_dir = Pathname(__FILE__).dirname
+lib_dir = base_dir + "lib"
+
+$LOAD_PATH.unshift(lib_dir.to_s)
+
+require "webhook-signer"
+
+run WebhookSigner::App.new

ansible/files/home/signer/webhook/lib/webhook-signer.rb

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+require "webhook-signer/app"

ansible/files/home/signer/webhook/lib/webhook-signer/app.rb

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+module WebhookSigner
+  class App
+    def call(env)
+      [200, {}, ["Hello Webhook Signer"]]
+    end
+  end
+end