Allow for specification of folder prefix for blob storage backups

Currently, S3 backups write directly into a `backups` and `data` folder from the root of the given bucket. The folder structure is done this way specifically to be more compatible and friendly with the S3 environment. However, this can be confusing in a shared environment where the `backups` or `data` folder prefixes are used for other purposes. Therefore, it could be useful to add a higher level folder prefix to contain the FDB backup info in the case we cannot or do not want to use an isolated bucket.

Start by locating the S3 backup implementation and its handling of the `backups` and `data` prefixes. Determine how a configurable higher-level prefix should be specified, then verify that backup objects are written beneath it while preserving the existing layout below that prefix.

Written by the indexing model from the issue text.