fcli * action: Add support for running ScanCentral and possibly other commands

[rsenden]

If we ever want to implement pipeline-style actions, we'd need to allow actions to run the scancentral package command, and possibly any other commands. We could either:

1. Provide a package: instruction that explicitly invokes the scancentral package command with any given arguments
2. Provide a more generic scancentral: instruction that allows for invoking any scancentral command
3. Provide a generic run: instruction for running arbitrary commands

Of course, we could also provide multiple of these options, like both a package: and run: instruction.

With options 1 and 2, the actions framework could potentially automatically download & install ScanCentral Client if it's not available yet, and potentially even remove this once the action terminates, or potentially these instructions could use an embedded ScanCentral Client. Of course, ideally, at some point packaging functionality should be incorporated into fcli.

With option 3, ScanCentral Client would already need to be installed, either manually by the user before running the action, or by the action itself by running fcli tool sc-client install before the run: step.