allow to customize AWS_SESSION_TOKEN env var name for role assume credentials

### What feature do you want to see added?

Currently the AWS session token is always bound to a fixed AWS_SESSION_TOKEN environment variable when a role is assumed. This adds a sessionTokenVariable option (mirroring the existing accessKeyVariable/secretKeyVariable config) so the session token can be bound to a custom environment variable name, falling back to AWS_SESSION_TOKEN when left blank.

### Upstream changes

_No response_

### Are you interested in contributing this feature?

yes, my pr is #334

Review the existing accessKeyVariable and secretKeyVariable configuration, then inspect pull request #334, which the issue identifies as the proposed implementation. Done means the session token can use a configured custom environment-variable name and falls back to AWS_SESSION_TOKEN when the option is blank.

Written by the indexing model from the issue text.