Implement EncryptedKey RetrievalMethod support

[hamir-suspect]

Implemented support for EncryptedKey RetrievalMethod.
Okta uses this format for example:

<saml2:EncryptedAssertion xmlns:saml2="urn:oasis:names:tc:SAML:2.0:assertion">
        <xenc:EncryptedData xmlns:xenc="http://www.w3.org/2001/04/xmlenc#" Id="_093f212499a589e38d740d491a3d6e6c" Type="http://www.w3.org/2001/04/xmlenc#Element">
            <xenc:EncryptionMethod xmlns:xenc="http://www.w3.org/2001/04/xmlenc#" Algorithm="http://www.w3.org/2001/04/xmlenc#aes256-cbc" />
            <ds:KeyInfo xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
                <ds:RetrievalMethod Type="http://www.w3.org/2001/04/xmlenc#EncryptedKey" URI="#_5a6e6cc3482245041c8929044504ce40" />
            </ds:KeyInfo>
            <xenc:CipherData xmlns:xenc="http://www.w3.org/2001/04/xmlenc#">
                <xenc:CipherValuexxx</xenc:CipherValue>
            </xenc:CipherData>
        </xenc:EncryptedData>
        <xenc:EncryptedKey xmlns:xenc="http://www.w3.org/2001/04/xmlenc#" Id="_5a6e6cc3482245041c8929044504ce40" >
            <xenc:EncryptionMethod xmlns:xenc="http://www.w3.org/2001/04/xmlenc#" Algorithm="http://www.w3.org/2001/04/xmlenc#rsa-oaep-mgf1p" >
                <ds:DigestMethod xmlns:ds="http://www.w3.org/2000/09/xmldsig#" Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
            </xenc:EncryptionMethod>
            <ds:KeyInfo xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
                <ds:X509Data>
                    <ds:X509Certificate>xxx</ds:X509Certificate>
                </ds:X509Data>
            </ds:KeyInfo>
            <xenc:CipherData xmlns:xenc="http://www.w3.org/2001/04/xmlenc#">
                <xenc:CipherValue>xxx</xenc:CipherValue>
            </xenc:CipherData>
            <xenc:ReferenceList>
                <xenc:DataReference URI="#_093f212499a589e38d740d491a3d6e6c" />
            </xenc:ReferenceList>
        </xenc:EncryptedKey>
    </saml2:EncryptedAssertion>

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