Added support for proxy attributes

[mdilena]

This commit adds support to proxy attributes by replacing the previous handling logic, which was just removing them from serialization.

CHANGES:
When a proxy attribute is found on the duplicated node, it'll be "un-proxied" -- which simply means that it'll be deleted and re-created as a standard dynamic attribute -- in order for the keys to be correctly serialized on disk.

Finally, when loading back the animation onto the actual proxy attribute, Maya will automatically wire things so the keys will end up onto the real attribute and not its proxy representation.

NOTE:
This has been used for almost a year in production without issues so far, so I figured it was safe to contribute back to the source repository for everyone to have access to the feature!

Fixes #366

[mdilena]

@krathjen can't seem to be able to add you as a reviewer, so pinging you for visibility just in case!

[krathjen]

Hey Matteo! Thanks for the PR. I really appreciate it. I’d like to implement a CLA workflow for the repo, but I’ll need a bit more time to set that up. I’ll circle back soon once it’s ready. Thanks for your patience!