`SdkBody` should always get re-exported

Currently, `SdkBody` is re-exported in services that have streaming, such as S3. However, services such as DynamoDB do not re-export it.

`SdkBody` is needed when writing any unit test that creates a `SdkError` since it has a raw response in it. For example:
```rust
SdkError::service_error(
PutItemError::ConditionalCheckFailedException(
ConditionalCheckFailedException::builder().build(),
),
http::Response::builder()
.status(some_status)
.body(SdkBody::empty()) // <-- here
.unwrap(),
)
```

Inspect how SdkBody is re-exported by streaming services such as S3, then compare the public exports for a non-streaming service such as DynamoDB. Apply the same export behavior to services that currently omit it, and verify that the shown SdkError test pattern can use SdkBody::empty().

Written by the indexing model from the issue text.