[FEAT] Add support for XML Signatures

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`.

Start by reading the related JSON signatures request in #122 and the XML Signatures specification example linked in this issue. Use `cyclonedx-cli sign` to create signed BOMs for testing; done means the Python library supports the XML signature component and can process those BOMs.

Written by the indexing model from the issue text.