feat(flightsql): allow configuring the driver memory allocator

[fallintoplace]

Fixes #59

Problem

The FlightSQL database driver always used memory.DefaultAllocator when building prepared-statement parameter records. Applications configuring the driver programmatically could not route these allocations through a checked, pooled, or otherwise custom allocator.

Change

Add an optional Allocator field to DriverConfig. Connector.Configure installs it on the FlightSQL client, and prepared-statement parameter builders use the client allocator.

Compatibility is unchanged:

• a nil allocator falls back to memory.DefaultAllocator
• DSN-based connections continue using the default because an allocator cannot be encoded in a connection string
• programmatic connector configuration can supply a custom allocator

Coverage

The prepared-statement test uses a concurrency-safe recording allocator and verifies that parameter construction allocates through the configured instance.

Validation

go test ./arrow/flight/flightsql/driver