Add a way to detect that an archive is zimit2

[Jaifroid]

Fixes first bullet point of #1199.

It works like this:

1. Assume all archives are 'open' as a baseline
2. If we find warc-headers, it must be zimit classic, so change zimType to 'zimit'. This is true currently, but might not be true in the future - so if headers are re-introduced, the logic will need to be modified, perhaps by testing the location of Wombat in the ZIM, but that is considerably more complicated to do.
3. If we don't find warc-headers, check if there is Scraper metadata. If the archive is not detected as 'zimit', but was scraped by warc2zim, then it must be zimit2.