Allow specifying a bucket prefix in bbs2gh

[romogo17]

Description

Right now, when migrating repositories from Bitbucket to GitHub, a blob storage provider is used as an intermediary step to upload the repository archive before GitHub imports it. This is provided by the --aws-bucket-name CLI argument.

However, all of the archives are uploaded to the root of the bucket. Since the archive names don't include any information about the projects and repository names, it's very difficult to know what project/repo a given archive maps to without (i) examining the logs, or (ii) downloading and examining the archives.

This issue is to request the addition of an optional CLI flag/argument to provide a bucket prefix (--aws-bucket-prefix). This would allow users to structure the archives in a way that makes more semantic sense as to what repo/project they were generated from or any other structure that makes sense (projects, repos, dates, etc)