Migration to aws-sdk-go v2

[wata727]

AWS has announced the end of support for aws-sdk-go v1, and we will need to migrate to v2.
https://aws.amazon.com/jp/blogs/developer/announcing-end-of-support-for-aws-sdk-for-go-v1-on-july-31-2025/

It would be relatively easy to simply update the Go module, but the problem is that code generation depends on v1 OpenAPI definitions. In v2, the API definition is written in Smithy, so we need to use this to rework the codegen.