fcli fod *-scan setup: Add --skip-if-exists option

[rsenden]

Enhancement Request

For pipeline integrations, you usually want to make sure that a scan has been set up properly to avoid scan requests from failing. However, if a scan setup already exists, we (usually) wouldn't want to override any existing setup. So, the *-scan setup commands should have a --skip-if-exists option (similar to app create and release create commands) that checks whether a scan setup already exists, and if so, skip the scan setup operation.

[rsenden]

Implemented for fcli fod sast-scan setup in 855d4f3, still needs to be implemented for other scan types. Ideally, this should be generic functionality (in a new AbstractFoDScanSetupCommand class, also see comments about the current AbstractFoDScanSetupCommand class in #598) to have a consistent implementation across scan types.

Note that for mobile, we can't check whether there's an existing scan setup (as FoD doesn't provide proper GET endpoint). If we do add a --skip-if-exists option (for example defined in a common superclass), help output should mention that this isn't available yet for mobile scans.