[ABC-464] Read and transfer Polymesh custom attributes

[SanaJal]

Purpose of this PR

Ticket/Jira #: ABC-464 and ABC-465

In this PR we handle the following :

• C++ side :
  • Reading the custom attribute :
    • template function to read the attributes : aiMeshSchema<T,U>::ReadAttribute .
    • struct to store all the data needed for the process of the attribute : AttributeData
  • Process the custom attributes
    • Add support of custom attributes in : -aiMeshSchema<T, U>::updateSummary() -aiMeshSchema<T, U>::readSampleBody() -aiMeshSchema<T, U>::cookSampleBody
    • Handle the different scopes : aiMeshSchema<T, U>::onTopologyChange
  • Transfer the attributes to C# :
    • Structure to transfer : struct AttributeDataToTransfer
    • copy_or_clear_vector(AttributeDataToTransfer dst[], const std::vector<AttributeData*>& src)
• C# side :
  • Define AttributeData stucture that corresponds to AttributeDatatoTransfer defined in C++
  • Allocate enough space for the custom attributes in AlembicMesh.AbcSyncDataBegin
  • For testing, we are processing the colors custom attributes and applying them to the meshes : AlembicMesh.ProcessData

This PR also contains also the code for ABC-486 - Get the number of custom attribute before filling the data . It made more sense to make this in the same PR as the way it was done before did not allow for testing the core feature (reading attribute and transferring them) without changing the hardcoded value in the code. There were also an error in the type we used to pass in the struct.

Testing

Functional Testing status:

Manual testing :

• import assets and inspect the data in C# once the vertex buffer is filled.
• import asset with color attributes (ex: attributeScopeCubes.zip), allow you to see the color applied to the meshes directly in the scene.

Performance Testing status:

N/A

Overall Product Risks

Complexity: Medium

Halo Effect:

Documentation & UX Writing

User facing text to review		Details
User interface	Y
Public API docs	Y
Changelog	Y

Documentation halo effect		Jira link
User manual
Other docs

Additional information

Note to reviewers:

Reminder:

• [ ] Add entry in CHANGELOG.md