Skip invalid containers with a warning

[BigRoy]

Changelog Description

Skip invalid containers with a warning - instead of breaking publishi…ng, loader, inventory, etc.

Additional review information

This avoids similar issues as #178 does - but is a 'fix' that would avoid the issue being problematic in existing scenes (instead of just on containerise) or if the container breaks in any other way.

The most likely case for these containers to become invalid was the issue #178 solves by 'bundling' undo so that users can't undo "half of the container creation" deleting only a few of its attributes. However, users can still break things in magical ways, time has shown us.

It would now log e.g. this if the container is incomplete and incomplete containers are not listed in the scene inventory either, because well they are incomplete:

// Warning: ayon_maya : Container 'char_hero_pointcacheMain_01__pointcacheMain_CON' is missing required keys: ['loader', 'representation']

Testing notes:

1. Load a few products
2. In outliner, enable: Display > Ignore "Hidden in outliner"
3. Select the containers
4. Randomly delete attributes on these instances, like loader, representation, etc.

A warning should be logged if any of the required keys ends up missing. The required keys are considered to be ["id", "name", "namespace", "loader", "representation"]

[LiborBatek]

Works well and when some attributes missing, it still publishes ok and just gives warning about some missing keys...LGTM

[LiborBatek]

@BigRoy should I also re-test it?