Handling serialization registration externally

Currently custom serialization registration must be done [within Distributed itself]( https://github.com/dask/distributed/blob/master/distributed/protocol/__init__.py ). However this is a bit tricky to manage (particularly for newcomers).

First they need to implement serialization for their libraries objects [like so]( https://github.com/rapidsai/cudf/blob/cc414f97002768e7bec926b77a6ec1ba58c5fada/python/cudf/cudf/comm/serialize.py#L9-L28 ). Second they need to add code to distributed to [register their serializers]( https://github.com/dask/distributed/blob/43ad2f8cfaa216588f15d39dc4ea24c58ac2c633/distributed/protocol/__init__.py#L104-L109 ). Third they need to test this somehow using a development install of their library and Distributed.

It would be useful if this registration (second) step could be done in external libraries. This would allow all of the changes in one place without requiring some coordinations of changes, PRs, and releases so as not to break anything.