[5.x] Implement Silenced interface

[driesvints]

This PR implements a new Silenced contract that, once applied to a job, will automatically silence it and move it to the Silenced tab. This is handy for packages so they can silence their jobs by default.

[freekmurze]

Instead of an interface, it could also be a property or function that controls this behaviour. That way package users could override the package default (you can't override an interface).

[driesvints]

@freekmurze problem is that the job isn't resolved at the time it gets marked as completed. We have a class string to inspect. We'd need to do some reflection magic I think in order to get that to work. Do you know how to do that?

[lioneaglesolutions]

@freekmurze @driesvints I just added #1238 - no idea if it's close enough but tests pass... happy to take feedback or close it if it isn't where it needs to be for the method override. I would assume you could do the same thing with a public property easily enough.