Protobuf support

[zZHorizonZz]

I think that since there is support for JSON, it might be worth offering support for Protocol Buffers (protobuf) as well. To my knowledge, Google Cloud Spanner is one of the few database services that currently supports protobuf natively (obviously as protobufs are made by google). In terms of implementation, this might be comparable to JSONB in SQL databases, as I imagine it would work in a similar way. The current approach requires manually deserializing protobuf messages and saving the values individually, or saving them as a bytestream, but neither option offers much flexibility.

References:
ZetaSQL Protocol Buffers
ZetaSQL Protocol Buffers Functions

[penberg]

@zZHorizonZz I think extensions/protobuf is something we could definitely merge to the tree and if it becomes popular enough we can consider bundling it by default.