USD: Add new get_prim_map API callable from python on_import hooks

When importing an USD, the Blender object names do not necessarily match
Prim names:

```
#usda 1.0
def Xform "xform1"
{
  def Mesh "MyObject"  # Will be imported as "MyObject"
}
def Xform "xform2"
{
  def Mesh "MyObject"  # Will be imported as "MyObject.001"
}
def Xform "xform2"
{
  def Mesh "MyObject"  # Will be imported as "MyObject.002"
}
```

This makes it difficult in the [USD Import Hooks]
(https://docs.blender.org/api/current/bpy.types.USDHook.html) to link a
Blender object back to its source Prim for additional processing. A
typical use cases for games is to generate UVs, create and apply a
material on the fly when importing a collision shape that does not have
a visual representation (hence no materials) based on some Prim
attributes, but that the artist needs to differenciate in Blender's
viewport.

The Import context exposes a new method `get_prim_map()` that returns a
`dict` of `prim path` / `list of Blender ID`.

For example, given the following USD scene,
```
/
 |--XformThenCube [def Xform]
 |   `--Cube [def Cube]
 |--XformThenXformCube [def Xform]
 |   `--XformIntermediate [def Xform]
 |       `--Cube [def Mesh]
 |--Cube [def Cube]
 `--Material [def Material]
     `--Principled_BSDF [def Shader]
```

the `get_prim_map()` method will return a map as:

```python
@static_method
def on_import(import_context):
  pprint(import_context.get_prim_map())
```

```json
{
  "/Cube": [bpy.data.objects["Cube.002"], bpy.data.meshes["Cube.002"]],
  "/XformThenCube": [bpy.data.objects["XformThenCube"]],
  "/XformThenCube/Cube": [bpy.data.objects["Cube"], bpy.data.meshes["Cube"]],
  "/XformThenXformCube": [bpy.data.objects["XformThenXformCube"]],
  "/XformThenXformCube/XformIntermediate": [bpy.data.objects["XformIntermediate"]],
  "/XformThenXformCube/XformIntermediate/Cube": [bpy.data.objects["Cube.001"], bpy.data.meshes["Cube.001"]],
  "/Material": [bpy.data.materials["Material"]],
}
```

Co-authored-by: Odréanne Breton <odreanne.breton@ubisoft.com>
Co-authored-by: Sttevan Carnali Joga <sttevan.carnali-joga@ubisoft.com>
Co-authored-by: Charles Flèche <charles.fleche@ubisoft.com>

doc/python_api/examples/bpy.types.USDHook.py

@@ -35,6 +35,8 @@ Hook function ``on_import()`` is called after the USD import finalizes. This fun
 as an argument an instance of an internally defined class ``USDSceneImportContext`` which provides the
 following accessors to the scene data:
 
+- ``get_prim_map()`` returns a dict where the key is an imported USD Prim path and the value a list of
+   the IDs created by the imported prim.
 - ``get_stage()`` returns the USD stage which was imported.
 
 The hook functions should return ``True`` on success or ``False`` if the operation was bypassed or