Add integration with the Key module

Integration with the Key module would allow site administrators to choose how the Authorize.net transaction key is stored—in a file outside the web root, for instance, or on another server altogether. This would allow for a greater level of security than storing the key in plaintext in the database.

The Key module is still in development, but I wanted to get the conversation started for how Commerce Authorize.Net could take advantage of its functionality. See the attached screenshot for an example of how choosing a key could appear in Commerce Authorize.Net's settings.