Backend/S3: Allow use of an alternative region for STS operations

[RLRabinowitz]

Summary

As part of accomplishing parity with the AWS provider, we should add support for an alternative region for STS operations with the S3 backend. Add an sts_region field,

• Add a sts_region field to the backend, in correspondence with AWS provider sts_region

References

• AWS provider code for reference - https://github.com/hashicorp/terraform-provider-aws/blob/3a33e810b7999e0f891c17940125831cf0ef49c2/internal/provider/provider.go#L229
• Related issue in Terraform Repository - backend/s3: Add parameter sts_region hashicorp/terraform#33693