Handle WARC filename conflicts with wb-manager add

[tw4l]

Description

Instead of not copying a WARC to the archive directory in case of a filename conflict, this PR instead modifies it to add the WARC with {original_name}-{dupe index}{extension} - i.e. example.warc.gz would be renamed to example-1.warc.gz, then example-2.warc.gz, and so on, until a filename that doesn't exist in the archive directory is found.

Motivation and Context

Should make for a more user-friendly experience of adding archives via wb-manager add.

Screenshots (if appropriate):

Types of changes

• Replay fix (fixes a replay specific issue)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added or updated tests to cover my changes.
• All new and existing tests passed.

[tw4l]

@ikreymer Would love to get your eyes on this to make sure I'm not missing any unintended side effects

[tw4l]

Approved offline at WAC!

[Quirinus]

Thanks!