Create stubbed API for FlightSQLClient

[asfimport]

Reporter: Tiffany Lam / @tifflhl

_{Note: This issue was originally created as ARROW-12923. Please see the migration documentation for further details.}

[asfimport]

David Li / @lidavidm:
@kylepbit @rafael-telles is this issue still relevant? (Also ARROW-12925, ARROW-12926)

[asfimport]

Todd Farmer / @toddfarmer:
This issue was last updated over 90 days ago, which may be an indication it is no longer being actively worked. To better reflect the current state, the issue is being unassigned. Please feel free to re-take assignment of the issue if it is being actively worked, or if you plan to start that work soon.

[lidavidm]

I don't think there needs to be a stub for the client.