[5.6] Pending Dispatch early dispatch

[dbpolito]

This is related to #23866 , which i didn't get much feedback unfortunately.

But looking at this PR that got merged today: #23890 , which is a pretty similar solution, i basically decided to give it a shot.

In summary, this is for when you call dispatch(), you can dispatch it when you want, without having to unset the variable for example.

Useful for when you are at php artisan tinker or when you want to fetch the return of the job handle or job id

[GrahamCampbell]

Hmm, this is missing protection against editing a job that's already been dispatched.

[tillkruss]

It's also missing tests.