Support AWS STS (AWS_SESSION_TOKEN) for S3 Object Storage

[BornToBeRoot]

Description of the change

Add sessionTokenKey to nextcloud.objectStore.s3.secretKeys once this https://github.com/nextcloud/server/issues/50579 is implemented.

Benefits

Support AWS STS for temporary credentials to access s3 object storage.

Possible drawbacks

-/-

Additional information

Currently, it is not possible to use dynamic, short-lived credentials with nextcloud to access s3...