Support for RetrievalMethod for encrypted assertions

[datb-com]

Please see datb-com fork of this for a 3 line modification to decodeEncryptedAssertion() that uses SimpleRetrievalMethodEncryptedKeyResolver to add support where an IDP (in this case Liferay7.4) provides encrypted assertions and specifies the EncryptionKey outside of the EncryptedData, referenced using RetrievalMethod. There is already a pull request open from a previous change (and i'm not a github expert) so please contact me for further info.