import existing CDX files

[anarcat]

Describe the solution you'd like

It should be possible to pass CDX files along with WARC files on wb-manager add. As things stand now, that command can take needlessly long as it creates its own CDX file - I already have that file, as it was generated during the crawl!

Describe alternatives you've considered

I know about the migrate-index command introduced in #80 (now cdx-convert?) but that's a separate command: after the wb-manager add command is ran, it's too late, and it's unclear if it can work before the add command is issued. In my tests, it only yields this message, regardless of what I feed it:

Index files up-to-date, nothing to convert

I also suspect custom user-defined collections might fit the bill, but I haven't figured out how to use those just yet.

Additional context

This, like #408, is a problem when processing large WARC files.