feat: Account specific default_region

[bbernays]

Summary:

Closes #1176

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[roneli]

#1180 I think this issue is relevant as well for ListBuckets as we have a us-east-1 hard coded in there so I would assume to access the default region should be used there as well instead of the hard coded value.

[hermanschaaf]

Some minor suggestions from me, but LGTM 👍