Refactor rsync::get to have data types and puppet-strings

[ekohl]

This uses the Puppet 7.21+ feature to pass a command as an array to simplify the invocation, and make it more secure. It also tightens data types and converts the docs to puppet-strings.

It's not complete, since I didn't convert the onlyif part. That still needs a shell I'm not sure how to properly deal with that.