Allow downloading arbitrary commits and artifacts in setup_multiversion. This is needed to allow fuzzer-self-tests to use this script for setting up its environment

• artifacts are signed, so we'll need to change boto3 to support signed URLs and pass in the key.
• we set the binary suffix based on the version passed in through the commandline; for arbitrary commits, we'll need to find the version from the evergreen project. For non-commit-versions, we don't want to use the evergreen project since people may want multiple commits for the same project, e.g. 4.2.2 and 4.2.1
• artifacts need to be stored alongside binaries, not in a separate directory.