Add download provider request support

[tinysec]

Summary

• expose registered download provider discovery by list and name
• correct BNPerformCustomRequest pointer and output-response ABI
• add safe synchronous GET and custom request wrappers with streaming input/output and progress callbacks
• capture managed callback exceptions across native calls and free native response metadata reliably

Verification

• dotnet build -c Release (0 warnings)
• 208/208 real-core E2E tests pass
• loopback HTTP E2E covers provider discovery, GET output, custom POST input, request/response headers, status code, and callback exception rethrow