Custom Handler for Mocking Reponses

[atran-r7]

With aws-sdk-go v1 I swapped out the default Send handler and used my own to mock responses from the client.
I'm trying to do this again for aws-sdk-go-v2 and see we can add custom middleware, but I'm having trouble understanding how to do so. I've tried adding a middleware after the Deserialize step, but it doesn't seem to let me modify the output response, only the input request. Is there a way I can do this?

My goal is to mock the Uploader in the features/s3/manager package. I initially wanted to use the recommended method of creating an interface and mocking that but unfortunately Uploader doesn't satisfy the interface because the method signatures are different. The interface is:

type Uploader interface {
	Upload(ctx context.Context, input *s3.PutObjectInput, opts ...func(*Uploader)) (*manager.UploadOutput, error)
}

but the Uploader type has the method:

Upload(ctx context.Context, input *s3.PutObjectInput, opts ...func(*Uploader)) (*UploadOutput, error)