Add configuration option for filebrowser S3 buckets to enable browsing buckets more conveniently in a cross-account setup

[paulmayer]

Description

Requesting a filebrowser-view with the root path s3a:// currently performs a ListBuckets operation which works fine if Hue's IAM principal is in the same account as the buckets that should be listed.
However, in cross-account setups (think data lakes that span buckets from multiple AWS accounts), ListBuckets will never be able to produce a desired list of buckets.

This enhancement proposes the introduction of a new filebrowser configuration option that takes a list of s3 buckets which will be returned at s3a:// (if set), while falling back onto ListBuckets otherwise.