Global: Validate unique subsets

[BigRoy]

Brief description

This implements a global validator whether the current publish will publish unique subsets (per asset).

This fixes #4330

Additional info

I've removed seemingly equivalent ones from Maya, Fusion and Photoshop - please double check.
There also appeared to be something relatively similar for Maya:

It would be good to know whether there are any cases where this logic doesn't make sense. Would we ever allow a single publish have multiple instances set to the same asset+subset. Can we think of a reason?

Testing notes:

I've only done a quick test in Maya.

1. Create some instances that publish to the same asset and subset names. (This should be invalidated!)
2. Create some instances that publish to different asset but match in subset names. (This should be allowed)
3. Create some instances that publish to same asset but differ in subset names. (This should be allowed)
4. Create some instances that publish to the same asset and subset names but during publishing deactive/disable the clashing ones. (This should be allowed)

[mkolar]

This makes good sense to me. I can't think of a scenario where it would be desirable to publish multiple instances into a same subset from one workfile.

[BigRoy]

I can't think of a scenario where it would be desirable to publish multiple instances into a same subset from one workfile.

Note that technically we're also talking about publishing from standalone publisher instead of 'a workfile' - not sure if that conceptually changes anything about potentially publishing into the same subset from separate instances?

[jakubjezek001]

I was testing on Nuke 12/13 at new and old publisher and all went smooth.