Support Sage Pay Form

[judgej]

Though mostly front end, Sage Pay Form shares a lot of the back end APIs and data structures with Server and Direct.

[judgej]

This API does not provide a securityKey with the result, so repeat payments are not possible with that result. However, fetching the transaction from the gateway should provide that detail, so we will need to implement fetchTransaction.

However, there are no plans for this at this time, as the fetchTransaction command uses the XML-based reporting and management API, which is a whole new thing largely out of scope in general for Omnipay.

[judgej]

This is now functional, though there are some additional message parameters to support for Form initiation only, and some tests for the encryption algorithms.

[judgej]

The encryption uses openssl and not PHP's mcrypt (which is deprecated as of PHP 7.1

[judgej]

The linked branch now supports Sage Pay Form if anyone else would like to try it out. It needs more tests, and there are some Form-only features and parameters to be added, but it is functional. It needs openssl to be available, as it does not have a fallback to mcrypt (this can be added if anyone wants to provide it - should be simple enough).