Return cryptomatte metadata in GetRenderStats

[sebastienblor]

Changes proposed in this pull request
When rendering cryptomatte with husk, we need the new cryptomatte attribute custom_output_driver to be enabled, so that cryptomatte metadatas are authored in the HdArnoldDriver. Then, we need to return this metadata to Hydra through the virtual function GetRenderStats() where we return a dictionary of values that will be authored in the exr metadata.

In order, to do this, in this PR we register all the drivers associated to a cryptomatte AOV (crypto_asset, crypto_masterial, crypto_object) and then in GetRenderStats we check the attribute custom_attributes and return the cryptomatte metadata in the dictionary.

I could have registered the drivers by their AtNode pointer instead of their name, to avoid doing a node lookup, but I'd rather avoid crashes if/when some nodes got deleted during IPR sessions. So I'm storing the driver names.
Also, for now I'm looping over all of these drivers, even though in theory all the drivers for cryptomatte AOVs will have the same data, so we could just check the first one. But since GetRenderStats doesn't seem to be called very often, it looks like it's not a bottleneck.

Issues fixed in this pull request
Fixes #1164