Support for API Certificate authenticate as well as Signature authentication

Issue Summary

I have a client which has a paypal account setup, but they are currently using Certificate Authentication on their other sites and not Signature Authentication. They've told me that Paypal does not support having a mixed method, so I'm either going to need to change all the rest of their sites to use Signature Authentication or change Drupal Commerce Paypal to use Certificate Authentication.

Here's how. Doesn't seem all that hard. In the rules configuration we simply need to allow the admins to choose a authentication method, then apply the appropriate credentials when sending the transaction.

Here's how to do it.
https://www.x.com/blogs/matt/2010/08/17/using-api-certificates-in-php