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ARROW-10487 [FlightRPC][C++] Header-based auth in clients

Added support for header based authentication in clients.
- Added support for base 64 encoded username / password auth to match Java implementation
- Added bearer token receiving and populating of call options to send back
- Added unit tests that connects C++ client and a mock C++ server to test authentication

Closes #8724 from lyndonb-bq/jduo/lyndon/flight-auth-cpp-redesign-client

Authored-by: Lyndon Bauto <lyndonb@bitquilltech.com>
Signed-off-by: David Li <li.davidm96@gmail.com>