filemanager: long-lived presigned URLs

[mmalenic]

The filemanager should support returning long-lived presigned URLs to objects (e.g. 7 days) in order to allow sharing data in a longer-term context. Currently the presigned URL length is limited by the credentials used in the Lambda role which doesn't last that long even if the presigned expiry is set to a higher limit.

To support this an IAM user that can issue longer presigned URLs can be created. The filemanager Lambda API function can then assume this user to create the long-lived presigned URLs.