Implement ProvideErrorMetadata for service errors (#3189)

This PR implements the `ProvideErrorMetadata` trait for service errors
as a prerequisite for implementing
[RFC-39](https://github.com/awslabs/smithy-rs/blob/main/design/src/rfcs/rfc0039_forward_compatible_errors.md).

Related SDK issue: https://github.com/awslabs/aws-sdk-rust/issues/780

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