Provide Protobuf event serialization protocol as alternative to JSON

At now Matrix use JSON format for event serialization, that contain uncompressed plain text messages and not so effective for traffic. Will be good to have alternative protocols for encode events body, like Protobuf, that use much less traffic, here is comparison of different message serialization protocols: https://eng.uber.com/trip-data-squeeze/

No files, tests, or entry points are named. Start by reading the Matrix protocol specification and the linked serialization comparison, then determine the scope and compatibility requirements for a Protobuf event format; done would require an agreed protocol design and corresponding specification changes.

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