Allow custom object key for artifacts instead of md5 hash

### Describe your idea/feature/enhancement

I don't know if it's a bug or not, but the s3 object key of the generated artifacts (using `sam package`) is the md5 hash of the zip file. On the documentation, I see that the name of the uploaded zip file is the same of the source folder, but I can't get it work in this way.

I need this, because I want to reference the uploaded object using a data source in Terraform (now I'm parsing the json output of `sam package`).
Furthermore, since the 2 keys are different (different hashes), `sam package` creates a new object instead of creating a new version of the older artifact.