backend/s3: Endpoint override parameter changes

[gdavison]

Changes to AWS API endpoint overrides.

For parity with the AWS Provider, move the endpoint configuration for the provider to the object attribute endpoints. Deprecate dynamodb_endpoint, iam_endpoint, endpoint (S3), and sts_endpoint and add endpoints.dynamodb, endpoints.iam, endpoints.s3, and endpoints.sts.

Environment variable updates are covered in #30479

[kmoe]

This looks reasonable. Thanks for enumerating the env vars and attributes.

[github-actions]

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