EMSUSD-1571 Manipulate prim schemas

[pierrebai-adsk]

Add new UFE helper functions:

• Add the getKnownSchemas helper function to retrieve all known schemas.
• Schemas are categorized by single and multi apply.
• Schemas are described by their plugin name and schema type name.
• The plugin names are optionally prettified, by default.
• Add the applySchemaToPrim and applyMultiSchemaToPrim helper functions.
• Add a getPrimAppliedSchemas helper function to retrieve the applied schemas of a prim.
• Add a getPrimsAppliedSchemas helper function to get all schemas of a set of prims.
• Add a findSchemasByTypeName helper function to find a schema.
• Expose the helper functions to Python.

Add unit tests:

• Add a unit test to verify the schemas helper function.
• Add unit tests for the schema application.

Add a command to manipulate prim schemas:

• Add a mayaUsdSchema command.
• Add undo/redo support to the schema command.
• Document the mayaUsdSchema command and its flags in the Readme.md.
• The -ufe flag to specify the prim UFE paths.
• The -app (-appliedSchemas) flag to retrieve the already applied schemas.
• The -sch (-schema) flag to specify which schema to add to the prims.
• The -in (-instanceName) flag to specify the instance name of a multi-apply schema.
• The -sas (-singleApplicationSchemas) flag to retrieve the list of known single-apply schemas.
• The -mas (-multiApplicationSchemas) flag tto retrieve the list of known multi-apply schemas.
• Add unit tests for the schema command.

Add schemas menu:

• Add a "Add Schema" menu item in the AE "Attributes" menu.
• Create sub-item for each schema, organized by plugin.
• Sort plugin and schema names to have consistent menu item order.
• Make sure to cleanup and prettify the plugin and schema names.
• Ask the user for the multi-apply schema instance name.
• Adjust callback metadata to have a nice undo entry name.