More options in vg cluster + make it usuable with current giraffe

[faithokamoto]

Changelog Entry

To be copied to the draft changelog by merger:

• Bring vg cluster up-to-date: now accepts GBZ files, can do short-read or long-read giraffe, and allows --prefix for better compatibility with vg autoindex
• Add some options to vg cluster to help with chaining issue diagnosis: print out cyclic snarl sizes, seeds with high hit amounts

Description

I forgot to PR this some time back, but here it is. This resolves #4610 while also doing a bunch of other stuff.