Add Support for STS Endpoints in AWS Non-Commercial (China / GovCloud) Partitions

### What feature do you want to see added?

When adding an AWS Credential to Jenkins that includes a Role ARN, the plugin will only use the commercial STS endpoint (sts.amazonaws.com) to retrieve temporary credentials. This means you cannot specify a Role ARN if you are supplying credentials from a non-commercial partition.

The feature should dynamically select the appropriate STS Endpoint based on the partition of the supplied IAM User.

**Note**: Simply supplying an IAM User from the China Partition works as expected.

### Upstream changes

_No response_