[FlightRPC] Make Handshake optional

[asfimport]

We should make it possible to not invoke Handshake for services that don't want it. Especially when using it with flight-grpc, where the standard gRPC authentication mechanisms don't know about Flight and try to authenticate the Handshake endpoint - it's easy to forget to configure this endpoint to bypass authentication.

Reporter: David Li / @lidavidm

Related issues:

• [FlightRPC] Handshake Extension (is related to)
• [FlightRPC] Authentication Redesign (is related to)

_{Note: This issue was originally created as ARROW-7579. Please see the migration documentation for further details.}

[asfimport]

David Li / @lidavidm: Not a blocker for 0.17, removing from fix versions.

[asfimport]

James Duong / @jduo: This can be closed when the authentication redesign from ARROW-9804 is ported to all server versions.

[asfimport]

Wes McKinney / @wesm: Any update on this?