Extend builder to allow for x509 issuer serial parameter specifiying

[lweberk]

@lweberk I'm happy to extend the builder for what I need as opposed to just exposing the raw functionality.

The only feature that is lacking for me (right now) is that I can't add the X509IssuerSerial block into the x509data.

 <dsig:X509IssuerSerial>
     <dsig:X509IssuerName>redacted</dsig:X509IssuerName>
     <dsig:X509SerialNumber>redacted</dsig:X509SerialNumber>
 </dsig:X509IssuerSerial>

Maybe we just add another option? x509issuer_serial(bool) (that defaults to false)?

doc.template()
        .canonicalization(XmlSecCanonicalizationMethod::ExclusiveC14N)
        .signature(XmlSecSignatureMethod::RsaSha1)
        .keyname(true)
        .keyvalue(true)
        .x509data(true)
        .x509issuer_serial(true)
        .uri("ReferencedID")
        .done();

All that issuer data is in the cert (obviously), which has already been given to the context. I'm not sure if xmlsec can get at it on its own as I am not very familiar with the library, I use https://docs.rs/openssl/latest/openssl/ for my use case.

Originally posted by @JoshuaNitschke in #5 (comment)

[lweberk]

@JoshuaNitschke would you like to push these changes (if you have them) upstream? While I'm already at it, I'd gladly accept a PR down this line.

[JoshuaNitschke]

@lweberk I never got to it unfortunately! I ended up doing a super hacky find and replace on the xml. 🥇