Extend the external data source framework with a request/response
Connector SPI for services that return data over a network protocol.
Implement the first connector for Apache Arrow Flight over gRPC.

- ConnectorFactory/Connector/ResultCursor: new SPI interfaces for
  connection-oriented external sources
- AsyncConnectorSourceOperatorFactory: async operator that drains a
  connector on a background thread with backpressure
- OperatorFactoryRegistry: dispatches ConnectorFactory via the
  ExternalSourceFactory hierarchy
- FlightConnectorFactory/FlightConnector: Arrow Flight implementation
  with schema resolution and streaming result cursor
- FlightTypeMapping: Arrow-to-ESQL type conversion
- GrpcDataSourcePlugin: plugin wiring via ServiceLoader
- EmployeeFlightServer: in-memory test fixture serving employee data
- FlightFormatSpecIT: integration tests via csv-spec against a live
  Flight server
- ExternalSourceDrainUtils: shared drain loop with timeout
- entitlement-policy.yaml: grants manage_threads and outbound_network

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