make prefix a required arg on AWSS3Source etc

[tmchartrand]

AWSS3Source, AWSS3Target, and GCPCloudStorageSource all have default=None for the prefix field. While the field should certainly be allowed to be empty (ie of type Optional[str]), sending an empty prefix is quite risky for a default behavior and can easily cause unintended consequences.
To prevent this, the default should be removed so that the user must specify default=None or default="" if that is actually desired.