feat(csharp): Add driver manager

[davidhcoe]

Implements C# driver manager matching adbc_driver_manager.h spec:

Core APIs:
• LoadDriver() / FindLoadDriver() - Load native/managed drivers
• OpenDatabaseFromProfile() - TOML profile support
• AdbcLoadFlags - Search control (env/user/system paths)

Features:
• Native + managed (.NET) driver loading
• TOML manifests with env_var() expansion
• Uses a custom TOML parser vs Tomlyn because a strong name is required for dependencies
• Cross-platform driver discovery
• Option merging (profile + explicit)
• Co-located manifest detection

Tests: 40+ unit tests + sample BigQuery/Snowflake implementations (attached, not checked in)

BigQueryDriverManagerTests.txt
SnowflakeDriverManagerTests.txt

[davidhcoe]

I debated whether to put this in its own assembly or as a subfolder under the shared Adbc project. I decided on a subfolder approach to reduce the number of dependencies required (especially since the AdbcDrivers are still using submodules and any tests there would require an additional submodule to be managed).