Workfile template build enhancements

[iLLiCiTiT]

Description

After discussion with @jakubjezek001 , @jrsndl , @ClementHector and @friquette we've made few conclusions about future of template workfile builder.

First of all current PR for Nuke must be updated to current openpype develop where abstractions were moved to different location. Easiest would be to create separated PR as the current one is based on Maya PR which was redone. This should ideally be updated and merged as soon as possible.

Secondly few point that must be changed or added to current implementation of workfile building which is already available in Maya:

• there must be option to have placeholder plugins for multiple types of placeholders to be able achieve different logic
• less abstraction on placeholder class
  • right now placeholder is very specific for loading and in very specific way
  • can be used only for loading but should be also able to call creation or different kind of loading (e.g. based on settings)
• placeholders plugins are defined by host thus a method which will return them should be added to host implementation
  • this should replace import template 'openpype.hosts.{host}.api.template_loader'

With these changes the placeholder plugins will be heart of building and logic can be modified by changing plugins.

My personal ideas

Use AttributeDefinitions instead of QArgparse.

• attribute definitionis can be used in any python, in headless or ui, and don't require import of Qt
  • they can be defined in placeholders so they're not "host specific" but "placeholder specific"
• they's missing "easy create" dialog which probably won't be as hard to do

[cuID:OP-3828]