Response Validation & Processing

- [x] Add Response Precondition checking
- [x] Add Response signature verification
- [x] Add Response Validation
- [X] Add Response Assertion Decryption
- [x] Add Response Assertion Validation
- [x] Add Response Assertion Transformers
- [ ] Test all the things
It indeed should be guarded, good catch, but not by a `has` - it should be guarded with an (pseudo) `if(encryptionEnabled)`. If encryption is required, a private key is required as well:
https://github.com/simplesamlphp/simplesamlphp/blob/master/modules/saml/lib/Message.php#L297
Leads to https://github.com/simplesamlphp/simplesamlphp/blob/master/lib/SimpleSAML/Utilities.php#L1458
which combined lead to that in the old config, if encryption is enabled, a privatekey without a prefix is required.