WIP NX-620: Support for offsite tarballs

Initial implementation and IT added.

All works, but is _very very fragile_ due to NX 2.x Core and Transport inability to fetch from custom URL. It (mis)uses mirror support, still, this is basically a hack.

Issue
https://issues.sonatype.org/browse/NX-620

CI
TBD
bah, leftover while i tried to figure out stuff, to be removed