Keda: Implement ScaledJob functionality for v2

[zroubalik]

Implement ScaledJob fucntionality for v2 as was discussed in https://github.com/kedacore/keda/issues/653

We should take into account requirements specific for scaling a Job and design ScaledJob CRD appropriately.

TODO

• Currently there's only a stub of the ScaledJob type which should be polished/improved and implement ScaledJob Controller functionality
• There are several issues opened for Job scaling, they should be taken into a consideration when implementing ScaledJob: https://github.com/kedacore/keda/issues?q=is%3Aissue+is%3Aopen+job
• Document Spec and behavior: https://github.com/kedacore/keda-docs/issues/163

[jeffhollan]

Let me chat with the CSE team that helped with this in v1 and has some customers using to see if they can help

[jeffhollan]

Chatted with CSE - they are finishing some hi-pri covid-19 projects so unable to jump in right now. Working with @anirudhgarg and @dimberman to see if we can have one of them help fund

[anirudhgarg]

Assigning to @TsuyoshiUshio to start having a look at this.

[TsuyoshiUshio]

I think I've done. the main part and big issues. I'll fix issue one by one.

[tomkerkhove]

It looks like we are missing migration docs as well or is it just me? https://keda.sh/docs/2.0/migration/#scaling-of-jobs