add new temporary backend method for RSA opaque key transition

Rationale for this is in https://github.com/pyca/cryptography/pull/1116#issuecomment-45442119
Something like

"This is a temporary additional backend method that returns a backend specific `RSAPrivateKey`. It will be removed in favor of changing `generate_rsa_private_key` after the `RSA*Numbers`  deprecation cycle is complete."

?