go/adbc/driver/flightsql: separate from the main project

### What feature or improvement would you like to see?

Since the core API definitions and the driver share a single go.mod, everyone downstream also picks up all transitive dependencies of the driver, even if they aren't actually using it. Some vulnerability scanners then pick up security issues in (e.g.) gRPC and warn you about it, even though your own project never actually picks up gRPC. (Other scanners analyze the final binary or call sites instead of just relying on go.mod/go.sum.) This is a tad annoying; perhaps we can separate the Flight SQL driver into its own go.mod to avoid this. We'd have to figure out how to do testing, etc. properly once they're separated, though.

Start in go/adbc/driver/flightsql and inspect the existing go.mod and its dependency relationship with the core API. Determine how the Flight SQL driver can use its own module while preserving repository testing, then verify that downstream users of the core API no longer inherit the driver's transitive dependencies.

Written by the indexing model from the issue text.