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ARROW-5643: [FlightRPC] Add ability to override SSL hostname checking

Adds the ability to override hostname checks, so you can connect to localhost over TLS but still verify that the certificate is for some other domain.

Example: when deploying on Kubernetes with headless services, clients connect directly to backend services and do load balancing themselves. Thus all instances of an application must present a certificate for the same hostname. To do health checks in such an environment, you can't connect to the TLS hostname (which may resolve to a different instance); you need to connect to localhost, and override the hostname check.

Also needs https://github.com/apache/arrow-testing/pull/5

Author: David Li <li.davidm96@gmail.com>

Closes #4608 from lihalite/flight-tls-java and squashes the following commits:

581fc7582 <David Li> Add ability to override SSL hostname checking