Refactor solution structure and add RabbitMQ config

Reorganized the solution structure to align with a layered architecture:
- Replaced `src` folder with `core`, `infrastructure`, and `presentation`.
- Moved projects to their respective folders.
- Added `DigitalData.MessagingService.Publisher.Abstraction` project.
- Removed `DigitalData.MessagingService.Client` project.

Updated project configurations and nesting in the solution file.

Added `appsettings.Secrets.json` with RabbitMQ and email account settings:
- RabbitMQ configuration includes hostname, port, credentials, and queue/exchange details.
- Email configuration includes SMTP server details and credentials.

src/core/DigitalData.MessagingService.Application/Common/Interfaces/IPdfProcessingService.cs

@@ -0,0 +1,26 @@
+namespace DigitalData.MessagingService.Application.Common.Interfaces;
+
+/// <summary>
+/// PDF processing service interface.
+/// Operates on streams instead of file paths for flexibility.
+/// </summary>
+public interface IPdfProcessingService
+{
+    /// <summary>
+    /// Validates if the provided stream contains a valid PDF document.
+    /// Throws InvalidPdfException if the stream is not a valid PDF.
+    /// </summary>
+    Task<bool> ValidatePdfAsync(Stream pdfStream, CancellationToken cancellationToken = default);
+    
+    /// <summary>
+    /// Extracts embedded files from PDF stream to the specified output directory.
+    /// Returns a list of paths to extracted files.
+    /// </summary>
+    Task<IEnumerable<string>> ExtractEmbeddedFilesAsync(Stream pdfStream, string outputDirectory, CancellationToken cancellationToken = default);
+    
+    /// <summary>
+    /// Gets the page count of the PDF document.
+    /// Throws InvalidPdfException if the stream is not a valid PDF.
+    /// </summary>
+    Task<int> GetPageCountAsync(Stream pdfStream, CancellationToken cancellationToken = default);
+}