Use ArrowFlight bearer token auth to create a session key for FlightSql clients

[avantgardnerio]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Since tables have to be registered per each session in Ballista, no one can query any tables unless we have a session identifier. The native Ballista wire protocol has a special field for this, but in FlightSQL the best we can do is use their bearer token auth.

Describe the solution you'd like

Accept a username and password on handshake, return a session token, use that token to maintain persistent sessions.

Describe alternatives you've considered

I can't really think of one.