Improve s3a region handling, including determining from endpoint

S3A region logic improved for better inference and
to be compatible with previous releases

1. If you are using an AWS S3 AccessPoint, its region is determined
from the ARN itself.
2. If fs.s3a.endpoint.region is set and non-empty, it is used.
3. If fs.s3a.endpoint is an s3.*.amazonaws.com url,
the region is determined by by parsing the URL
Note: vpce endpoints are not handled by this.
4. If fs.s3a.endpoint.region==null, and none could be determined
from the endpoint, use us-east-2 as default.
5. If fs.s3a.endpoint.region=="" then it is handed off to
The default AWS SDK resolution process.

Consult the AWS SDK documentation for the details on its resolution
process, knowing that it is complicated and may use environment variables,
entries in ~/.aws/config, IAM instance information within
EC2 deployments and possibly even JSON resources on the classpath.
Put differently: it is somewhat brittle across deployments.