backend/s3: add ec2_metadata_service_endpoint argument

[jar-b]

Adds ec2_metadata_service_endpoint and ec2_metadata_service_endpoint_mode arguments to the S3 backend for parity with the AWS Provider.

% TF_ACC=1 go test -count=1 ./internal/backend/remote-state/s3/...
ok      github.com/hashicorp/terraform/internal/backend/remote-state/s3 107.609s

Closes #30444
Relates #33687

Target Release

1.6.x

Draft CHANGELOG entry

UPGRADE NOTES

• Adds ec2_metadata_service_endpoint and ec2_metadata_service_endpoint_mode arguments as well as support for the corresponding AWS environment variables, AWS_EC2_METADATA_SERVICE_ENDPOINT and AWS_EC2_METADATA_SERVICE_ENDPOINT_MODE. The environment variable AWS_METADATA_URL is also supported for compatibility with the AWS provider, but is deprecated. (#30444)

[jar-b]

Closing in favor of #33765, which groups the remaining AWS provider parity enhancements into a single PR.

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