[12.x] Implement invoice renderer

[driesvints]

This PR implements a way to use a different renderer for invoices in Cashier. This way, more powerful renderers like Browsershot can be used.

This PR is fully backwards compatible with current code. Additionally it:

• Adds a new option to set a custom renderer that's resolved from the container
• Together with some options that can be passed to the renderer
• Moves all dompdf functionality to its dedicated DompdfInvoiceRenderer class
• Binds that class to the new InvoiceRenderer contract by default

@taylorotwell I'll need your help with the config option description 😅

[driesvints]

Please note that tests are failing due to rate limiting in Stripe.