Add type annotations to avoid reflection

I did some profiling of our app with Yourkit and found that we were spending a great deal of time doing reflection on some of the S3 api methods. It seems that the type tag on s3-client wasn't having any effect. I added type annotations at each call to s3-client in order to avoid this.

Now *warn-on-reflection* doesn't report anything from aws.sdk.s3 and we don't see reflection calls as a hot spot in our app anymore.