Support loading multiple products to one node

[BigRoy]

Is there an existing issue for this?

• I have searched the existing issues and added correct labels.

Please describe the feature you have in mind and explain what the current shortcomings are?

With #13 we can now load and manage matteshapes and tracker points - however, it assumes that a single node will only ever get one loaded product at most. Even though it may be nicer to allow e.g. importing BOTH tracker points and matte shapes to a single roto node and manage them.

Suggested implementation?

Store the instances as a list of ayon_instances metadata on the nodes. In that metadata also keep track of which objects on the node belong to that particular instance, so that we don't need to remove all child objects on update - but only replace those entries that are tracked and go from there.

Describe alternatives you've considered:

No response