Expose Maya USD creator that creates a static model product type

[BigRoy]

Changelog Description

Exposes a dedicated creator that exports a Maya USD export as a "model" product type.

Additional review information

TL;DR

We have a long history of a model product type creator in Maya, focused on exporting static geometry to Maya Scene and Alembic. Now we also want to include USD exports, but also allow the Maya USD contribution workflow to trigger, etc.

This is one potential approach to do so.

Detailed

This now allows to publish USD products with USD contributions as a "model" product type, e.g. a modelMain as part of the usdAsset's model layer and also have it trigger the model product type validations.

It's good to know however that a few validators will need to be disabled in settings for this product type to make sense (otherwise it wouldn't pass the model validations).

In particular disable:

ayon+settings://maya/publish/ValidateNoNamespace
ayon+settings://maya/publish/ValidateTransformNamingSuffix

This is because the top group should be the asset name without suffix (so it should not be e.g. _GRP suffix) and the creators' "asset template hierarchy" it creates starts with a namespace to allow publishing multiple model asset contributions separately from the one scene (without having a group name conflict between the two groups in the scene).

Also, if ONLY the USD should be exported from this product type instead of also the Maya Scene and Alembic, then disable the extractors:

ayon+settings://maya/publish/ExtractModel
ayon+settings://maya/publish/ExtractAlembic/families <- remove the model family

There happened to also be a way to export a USD previously from the model family which should also be disabled:

ayon+settings://maya/publish/ExtractMayaUsdModel

That behavior where the existing model creator would export USD however had the issue that:

• The USD Contribution workflow wouldn't trigger, because it triggers on usd family - not on model product type.
• The creator doesn't expose the "Create Asset Template Hierarchy" pre-create feature to simplify creating the correct USD asset structure in the scene outliner (correct grouping)
• The creator doesn't expose all relevant MayaUSD export settings toggles - however, one could argue that maybe those shouldn't have been Creator attributes to begin with.

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Fix #207

Testing notes:

1. Publish the special Maya USD Model product type with asset contribution workflow, etc.