Support explicitly declared variable length offsets

[rien333]

A <ByteSequence> without a Reference=... attribute was previously assumed to implicitly indicate a variable length offset; however, variable length offsets may also be declared explicitly using Reference="Variable". The only file format that currently uses this explicit form is WACZ (fmt/1840):

<!-- from https://cdn.nationalarchives.gov.uk/documents/container-signature-20240715.xml --> 
<ContainerSignature Id="80000" ContainerType="ZIP">
    <Description>Web Archive Collection Zipped</Description>
    <Files>
        <File>
            <Path>datapackage.json</Path>
            <BinarySignatures>
                <InternalSignatureCollection>
                    <InternalSignature ID="80000">
                        <ByteSequence Reference="Variable">
                            <SubSequence Position="1">
                                <Sequence>'wacz_version'</Sequence>
                            </SubSequence>
                        </ByteSequence>
              ...

With this commit, siegfried can now thus differentiate WACZ from ZIP files — provided, at least, that you are also using a container signature file released in 2024 or later.

I can take a look at adding support for WACZ files in siegried later as well, since both WACZ and siegfried are important for some projects I'm working on!

[richardlehane]

Thanks for this PR Rijnder, I check and merge shortly ... [edit] & WACZ support would be very welcome! Siegfried's warc/arc decoding is all in this repo: https://github.com/richardlehane/webarchive