Backups to S3: specify a path/prefix/folder

In the code where you configure backups to S3 it would be nice if you could specify a path/prefix/folder. (That is confusing because I have seen folders referred to by different terms in S3 GUI and docs.)

Use cases:

· Store files from multiple gitlab installations in a bucket while keeping it clear which is from what install.

· All different tools to store backups in the same bucket in a remote region for backups in case of catastrophic failure of an AWS region.

· Allows finer grain control of lifecycle specs for a shared bucket.

Links / references

https://gitlab.zendesk.com/agent/tickets/42800