[FEAT] Add support for XML Signatures

[makew0rld]

Extracted from #850

Just as #122 requests support for JSON signatures (JSF), this ticket requests support for the equivalent spec component in XML: XML Signatures.

Example from the spec:

For testing, XML signatures can be added to BOMs with cyclonedx-cli sign.

[jkowalleck]

XML signing is not part if the CycloneDX implementation, instead, it is intended to be done via https://www.w3.org/TR/xmldsig-core/

[makew0rld]

I'm not sure what this means. XML Signatures are part of the CycloneDX spec, even though a different spec is referenced for how to implement it. It's same with JSF.

That's why cyclonedx-cli supports XML Signatures, for example.