Add optional S3_REGION config

[lognaturel]

We originally decided not to make region configurable because minio detects, caches and uses the region when making a request against a generic storage URL.

We've now discovered that for Amazon S3, regions introduced after 2019 like af-south-1 can't use the generic URL: https://forum.getodk.org/t/using-external-aws-s3-bucket-in-region-other-than-us-east-1/51392