[FEATURE REQUEST] Add ability to S3FS to limit to specific directory

Is your feature request related to a problem? Please describe.
When using a shared S3 bucket for S3FS within an org it is beneficial to limit the path that is searched/cache by the Salt master. The idea here being that we could use this similar to gitfs where you can specify the file path in the repo as the starting point for the environment.

Describe the solution you'd like
The current configuration looks like this:

s3.buckets:
  production:
    - bucket1
    - bucket2
  staging:
    - bucket3
    - bucket4

It would be nice to be able to specify something like this:

s3.buckets:
  production:
    - bucket1:/path/to/start
    - bucket2:/
  staging:
    - bucket3:/path/to/start
    - bucket4:/

Or to have an option like gitfs_root.

Start with the s3.buckets configuration shown in the issue and compare its requested path scoping with the linked gitfs_root documentation. Trace how bucket entries are interpreted and define completion as supporting per-bucket starting paths without changing the existing bucket-only configuration.