Refactor DIExtensions and add exception handling middleware

- Improved documentation in DIExtensions.cs for clarity.
- Added project reference to DigitalData.Core.Exceptions.
- Updated solution file to include DigitalData.Core.Exceptions.
- Introduced ExceptionHandlingMiddleware for global exception handling.
- Added BadRequestException and NotFoundException classes.
- Created DigitalData.Core.Exceptions project with .NET 7.0, 8.0, and 9.0 support.

DigitalData.Core.Exceptions/NotFoundException.cs

@@ -0,0 +1,22 @@
+namespace DigitalData.Core.Exceptions;
+
+/// <summary>
+/// Represents an exception that is thrown when a requested resource is not found.
+/// </summary>
+public class NotFoundException : Exception
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="NotFoundException"/> class.
+    /// </summary>
+    public NotFoundException()
+    {
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="NotFoundException"/> class with a specified error message.
+    /// </summary>
+    /// <param name="message">The message that describes the error.</param>
+    public NotFoundException(string? message) : base(message)
+    {
+    }
+}