FlightSqlService trait does not allow impls to do handshake

[avantgardnerio]

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

Apache Arrow Ballista now supports FlightSQL, and can run simple statements like select 1;, however due to its architecture, tables need to be registered before any substantive queries can be run. Unfortunately, tables have to be registered on a context, or else the next query will have no idea that they were registered. The native Ballista protocol uses a custom field to implement this behavior, but for FlightSQL, we should use its native method. However the existing FlightSqlService trait forces this to return an unimplemented Err.

Describe the solution you'd like

It seems reasonable to me to extend the FlightSqlService trait proxying the raw method signature from the FlighService trait.

Describe alternatives you've considered

I don't know enough about Rust to envision another approach.