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ARROW-5979: [FlightRPC] Expose opaque (de)serialization of protocol types

Motivation: make it easier for non-Flight services to consume/produce Flight types. For instance, a REST service may want to produce FlightInfo or Ticket objects for a Flight client. This gives us an (opaque) serialization of a few protocol types that can be deserialized and then used as normal.

I've noticed some of the Java APIs accidentally expose conversions to/from Protobuf types; if it's worth it, I can go back and make those private.

Personal Travis: https://travis-ci.com/lihalite/arrow/builds/119836432

Closes #4914 from lihalite/arrow-5979 and squashes the following commits:

14cc13bd5 <David Li> Expose Flight protocol types in Python
c17160af8 <David Li> Expose Flight protocol types in C++
5662c4a20 <David Li> Expose Flight protocol types in Java

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>