Add DuckDB FlightSQL sample connector (ADBC)

[ryan-syed]

Summary

Adds a new sample connector for DuckDB using the FlightSQL protocol via ADBC (Arrow Database Connectivity). This is the first ADBC-based connector sample in the repository, intended as a reference implementation for partners building FlightSQL connectors for Power BI.

What's included

Connector source:

• Adbc.Connection-based connectivity with FlightSQL gRPC transport
• Full query folding via SqlView.Generator with DuckDB SQL dialect overrides
• Navigation tree (Database > Schema > Table/View) built from information_schema
• Primary/foreign key detection for automatic Power BI relationship creation
• Support for 24 DuckDB types including HUGEINT, UUID, JSON, ARRAY, STRUCT
• Three authentication modes: Username/Password, Bearer Token, Anonymous
• DirectQuery and Native Query support

SQL Generator architecture:

• SqlGeneratorCommon.pqm: Shared SQL92 base infrastructure (reusable by other ADBC connectors)
• SqlGenerator.pqm: DuckDB-specific overrides (LIMIT/OFFSET, function remapping, typed literals, type facets)

Test suite:

• 200+ standard query folding and functional tests
• Datasource-specific tests for extended types (HUGEINT, UUID, JSON, BLOB) and nested types (ARRAY, STRUCT)
• Performance tests over 10,000-row NYC Taxi dataset
• Docker-based test environment using SQLFlite
• Setup scripts and guides for running tests locally

Notes

• The FlightSQL ADBC driver ships with Power Query SDK Tools and Power BI Desktop; no separate driver installation is needed
• DuckDB logo icons are not included
• Test credentials in Tests/Credentials/ are dummy values for the local Docker environment