Add feature ID for S3 Transfer Manager (#3921)

## Motivation and Context
This PR introduces the `AwsSdkFeature` enum, which will be used for AWS
SDK-specific feature identifiers. For now, it includes a single ID for
the S3 Transfer Manager. This approach is similar to the existing
[SmithySdkFeature](https://github.com/smithy-lang/smithy-rs/blob/aac9becfd469e2479f68c61fd4c8074ddf755482/rust-runtime/aws-smithy-runtime/src/client/sdk_feature.rs#L10)
in terms of its module and crate structure (that is for generic client
but `AwsSdkFeature` is AWS specific); the `sdk_feature` module is hidden
as it is intended for internal use only.

## Testing
Added an integration test to verify tracking a business metric for
Transfer Manager. Since Transfer Manager is a high-level library, we do
not track its business metric directly within the `smithy-rs`
repository. Instead, [an s3
client](https://github.com/awslabs/aws-s3-transfer-manager-rs/blob/main/aws-s3-transfer-manager/src/config.rs#L21)
used in
[aws-s3-transfer-manager-rs](https://github.com/awslabs/aws-s3-transfer-manager-rs

)
can configure the s3 client as demonstrated in the integration test to
track the metric.

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Co-authored-by: Zelda Hessler <zhessler@amazon.com>