Add streaming support and improve JSON serialization

- Updated `Response` record class to allow nullable `Question` and `Answer` properties; moved `StreamState` enum to a new file.
- Added `JsonStringEnumConverter` in `Program.cs` for better enum serialization.
- Corrected terminology in document upload endpoint description.
- Introduced `/api/ask-streaming` endpoint for streaming question responses.
- Added `AskStreamingAsync` method in `VectorSearchService` for handling streaming logic.
- Created `StreamState.cs` to define `StreamState` enum with `Start`, `Append`, and `End` values.

SqlDatabaseVectorSearch/Models/StreamState.cs

@@ -0,0 +1,8 @@
+namespace SqlDatabaseVectorSearch.Models;
+
+public enum StreamState
+{
+    Start,
+    Append,
+    End
+}