Upgrade to Protobuf 4.x

[clairemcginty]

As of Beam 2.63, GCP libraries bom is using Protobuf 4. Beam itself is still mostly using 3, and Scio is still using 3; our IO integration tests have continued to pass, indicating they are backwards-compatible enough to mix for the time being.

On the Scio side, we're blocked from upgrading to 4 until we can figure out a solution for Tensorflow, which is still on 3 and doesn't have any immediate plans to upgrade. Likely we'll have to add an SBT task to download and recompile core Tensorflow protos like Example and Feature and compile them with Protobuf 4.