feat: Add OAuth2 server and client implementation with PKCE support

- Implemented OAuth2 server with client registration, authorization, and token endpoints.
- Created HTML templates for client authorization, client creation, and client editing.
- Developed an OAuth2 client application using Hono.js and Bun, supporting authorization code grant flow.
- Integrated PKCE (Proof Key for Code Exchange) for enhanced security during authorization.
- Added session management using cookies for user authentication.
- Included detailed README documentation for setup and usage instructions.

example-oauth2-server/website/templates/authorize.html

@@ -0,0 +1,22 @@
+<p>The application <strong>{{grant.client.client_name}}</strong> is requesting:
+<strong>{{ grant.request.scope }}</strong>
+</p>
+
+<p>
+  from You - a.k.a. <strong>{{ user.username }}</strong>
+</p>
+
+<form action="" method="post">
+  <label>
+    <input type="checkbox" name="confirm">
+    <span>Consent?</span>
+  </label>
+  {% if not user %}
+  <p>You haven't logged in. Log in with:</p>
+  <div>
+    <input type="text" name="username">
+  </div>
+  {% endif %}
+  <br>
+  <button>Submit</button>
+</form>