MultiKueue: support mutating the priority class for workloads managed by MultiKueue

[mimowo]

What would you like to be added:

Recently we introduced this feature #7289.

However, I expect this is not working well currently as explained here: #7289 (comment)

Also, for MultiKueue we need to solve another prerequisite issue: #7350

Why is this needed:

To allow the functionality of mutating the workload priority also when MultiKueue is used.

[mimowo]

cc @mwysokin @mwielgus @mbobrovskyi