SC-SAST: Implement 'sc-sast scan prepare' command

[rsenden]

This should be based on the scancentral package command. Initially, we would require ScanCentral Client to be installed, with the 'sc-sast scan prepare' command simply being a wrapper for the scancentral package command. Eventually, packaging functionality should be natively integrated into fcli.

[Grisolfi]

That will be awesome to have. @rsenden

[rsenden]

Merging with #82, as implementation will be very similar.