Add session token so we can register tables for FlightSQL

[avantgardnerio]

Which issue does this PR close?

Closes #112.

Rationale for this change

Since tables have to be registered to sessions, we need a session identifier or else each new FlightSql command runs on a clean slate with no tables. To fix this, we need a session identifier.

What changes are included in this PR?

Implementation of a Basic Auth handshake, returning a UUID bearer token that the FlightSqlClient should send back with each request. Using this, I was successfully able to register the TPC-H customer table and select some data from it.

Are there any user-facing changes?

They can start doing interesting things with JDBC / ODBC.