Implement `StdError::source()` for Error enum (#2564)

## Motivation and Context
This is an attempt at fixing
https://github.com/awslabs/aws-sdk-rust/issues/784.

The service-level `Error` enum implements `std::error::Error` but does
not implement its `source()` method. This means that an error library
like `anyhow` or `eyre` won't be able to display the root cause of an
error, which is especially problematic for the `Unhandled` variant.

## Description
I modified `ServiceErrorGenerator` in the `codegen-client` crate and
replaced the line that output `impl std::error::Error for Error {}` with
an impl block that implements the `source()` method by delegating to the
inner error structure.

## Testing
I've added a simple unit test to `ServiceErrorGeneratorTest`.

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._