Allow extensions

[Weasy666]

This PR implements the Extensions struct and adds it to the AuthnRequest. This allows to create requests with arbitrary extensions, like this:

<?xml version="1.0" encoding="UTF-8"?>
<saml2p:AuthnRequest xmlns:saml2p="urn:oasis:names:tc:SAML:2.0:protocol" ID="..."
    Version="2.0" IssueInstant="2024-04-10T15:24:12.152Z"
    Destination="..."
    ForceAuthn="false" ProtocolBinding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-POST"
    AssertionConsumerServiceURL="...">
    <saml2:Issuer xmlns:saml2="urn:oasis:names:tc:SAML:2.0:assertion"
        Format="urn:oasis:names:tc:SAML:2.0:nameid-format:entity">https://examplecom</saml2:Issuer>
    <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
        <ds:SignedInfo>
            <ds:CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
            <ds:SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256" />
            <ds:Reference URI="...">
                <ds:Transforms>
                    <ds:Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" />
                    <ds:Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
                </ds:Transforms>
                <ds:DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
                <ds:DigestValue>...</ds:DigestValue>
            </ds:Reference>
        </ds:SignedInfo>
        <ds:SignatureValue>...</ds:SignatureValue>
        <ds:KeyInfo>
            <ds:X509Data>
                <ds:X509Certificate>
                ...
                </ds:X509Certificate>
            </ds:X509Data>
        </ds:KeyInfo>
    </ds:Signature>
    <saml2p:Extensions>
        <example:AuthenticationRequest xmlns:example="https://www.example.com/request/2020/09" Version="2">
            <example:RequestedAttributes>
                <example:RequestedAttribute Name="urn:name1"
                    RequiredAttribute="false" />
                <example:RequestedAttribute Name="urn:name2"
                    RequiredAttribute="false" />
                <example:RequestedAttribute Name="urn:name2"
                    RequiredAttribute="false" />
            </example:RequestedAttributes>
            <example:DisplayInformation>
                <ui:Version  xmlns:ui="https://www.example.com/request/2020/09/ui/v1">
                    <ui:DisplayName>Fancy name</ui:DisplayName>
                </ui:Version>
            </example:DisplayInformation>
        </example:AuthenticationRequest>
    </saml2p:Extensions>
    <saml2p:NameIDPolicy AllowCreate="true" />
</saml2p:AuthnRequest>

I am not exactly happy with using a Vec<String> as internal data for the Extensions struct, which means the extensions can only be serialized. Deserialization does not work with how quick-xml is working currently, because we don't know beforehand which name each single extension element has and we are also not able to add a field with "dynamic" objects quick-xml does not know how to deserialize this. It also does not have the ability to just deserialize the elements into raw strings. There is an issue for that, see tafia/quick-xml#257, but it's not implemented right now.

[Weasy666]

I think i've found a better solution. Will update this in the coming weeks