Browse cross account s3 buckets

## Problem
In bigger organizations, S3 buckets often scattered across multiple AWS accounts, for example when data lake is managed centrally, while workloads are running elsewhere. Currently, the extension is not able to list and browse buckets across accounts, for example by explicitly providing an existing bucket name.


## Expected behavior
Assuming that IAM policy permits, it would be great to be able to connect to cross-account buckets. For example, in IntelliJ BigData Tools extension one can specify the bucket name explicitly in the `custom roots` section


Start by locating the extension's S3 bucket-listing flow and the connection or custom-roots settings described in the issue. Review how account and IAM context are handled, then define the behavior for explicitly named cross-account buckets; done means permitted buckets can be connected, listed, and browsed.

Written by the indexing model from the issue text.