Add Polar payments integration

[cameronapak]

Description:

I’d like to add support for Polar payments to this project. My goal is to extend the existing Hono routes using Bknd so that users can make payments via Polar.

Acceptance Criteria:

• Extend Bknd's Hono routes to support payment endpoints.
• Integrate with Polar’s payment API.
• If I have POLAR_PAYMENT_URL, then this route is added. Otherwise, it's not added.
• Provide clear documentation and example usage in the README.md

Here's an example of what it looks like:

[Lftobs]

nice