[9.x] Adds closure support to dispatch conditionals.

[DarkGhostHunter]

What?

Enables usage of closures to dispatch a job conditionally. The closure will receive the job arguments.

MyQueuableJob::dispatchIf(Something::check(...), $name);

Also adds tests both dispatchIf and dispatchUnless.

[taylorotwell]

@DarkGhostHunter

IMO the entire job instance should be passed to the closure. I have updated the PR. Thoughts?

[DarkGhostHunter]

I don't have too much of maneuverability if I get the job instance full of non-public members, so it would force me to:

a) make a public method for additional checks.
b) make its properties public.

Since these are small chores, I can see it can be more convenient and forces me to push part of the check in the instance itself.

I approve these changes.