Add support for assuming a role arn with sts

[icholy]

This adds a role_arn config option which uses sts:Assume to create temporary credentials for the s3 operations.

[CLAassistant]

All committers have signed the CLA.

[icholy]

ruff is complaining that the _prepare_credentials method is too complex. I extracted my code into a method and I don't think there's any way for me to make it shorter without refactoring surrounding code. But that feels out-of-scope for this PR.

[skshetry]

@icholy, can this be upstreamed to s3fs? If it is possible, I'd prefer that and then wire the support here and/or in dvc.

[icholy]

@skshetry I'll see if they're open to it.

[skshetry]

Thanks @icholy. One more thing, s3fs and DVC uses aiobotocore instead of botocore, so if the aiobotocore provides async version of the APIs, then that'd be preferred.