ARROW-8294: [Flight] Add DoExchange to Flight.proto

This is the proposed addition of the DoExchange endpoint to Flight for the mailing list discussion.

Closes #6686 from lidavidm/doexchange-format

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Wes McKinney <wesm+git@apache.org>

format/Flight.proto

@@ -87,6 +87,15 @@ service FlightService {
    */
   rpc DoPut(stream FlightData) returns (stream PutResult) {}
 
+  /*
+   * Open a bidirectional data channel for a given descriptor. This
+   * allows clients to send and receive arbitrary Arrow data and
+   * application-specific metadata in a single logical stream. In
+   * contrast to DoGet/DoPut, this is more suited for clients
+   * offloading computation (rather than storage) to a Flight service.
+   */
+  rpc DoExchange(stream FlightData) returns (stream FlightData) {}
+
   /*
    * Flight services can support an arbitrary number of simple actions in
    * addition to the possible ListFlights, GetFlightInfo, DoGet, DoPut