RFC30: serde support for error metadata

[thomas-k-cameron]

Motivation and Context

This PR implements serde support for ErrorMetaData.

Description

Serde is implemented behind a feature gate.
This PR do not introduce breaking changes.

Prerequisite PRs

You can reduce the diff if you merge them first.

• Add RUSTFLAGS = aws_sdk_unstable as environment variable when running Cargo from kotlin #2614

Parent PR

This PR is listed as one of prerequisite PRs on this PR.

• RFC30 #2615

Testing

1. It checks whether serialized/de-Serialized results are the same.
2. Checks whether it can de-serialization of JSON file produces an expected result.

Checklist

• I have updated CHANGELOG.next.toml if I made changes to the smithy-rs codegen or runtime crates
• I have updated CHANGELOG.next.toml if I made changes to the AWS SDK, generated SDK code, or SDK runtime crates

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[jdisanti]

Can you update the PR description to say why ErrorMetadata needs ser/de support?

[thomas-k-cameron]

Can you update the PR description to say why ErrorMetadata needs ser/de support?

Sorry, I realized that there was a mistake in my implementation.

So, I added serde to ErrorMetadata because it the compiler was complaining to me.
I assumed that some input/output type was using ErrorMetaData, but I realized that it was because I was mistakenly adding serde traits to ErrorType.

I'm closing this PR.