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.Domain/DigitalData.MessagingService.Domain.csproj

@@ -0,0 +1,14 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+	<TargetFrameworks>net462;net8.0</TargetFrameworks>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <LangVersion>latest</LangVersion>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="MediatR" Version="12.2.0" />
+  </ItemGroup>
+
+</Project>

src/core/DigitalData.MessagingService.Domain/Exceptions/AuthenticationFailedException.cs

@@ -0,0 +1,16 @@
+namespace DigitalData.MessagingService.Domain.Exceptions
+{
+    /// <summary>
+    /// Exception thrown when OAuth2 authentication fails.
+    /// </summary>
+    public class AuthenticationFailedException : Exception
+    {
+        public AuthenticationFailedException(string message) : base(message)
+        {
+        }
+
+        public AuthenticationFailedException(string message, Exception innerException) : base(message, innerException)
+        {
+        }
+    }
+}

src/core/DigitalData.MessagingService.Domain/Exceptions/NotFoundException.cs

@@ -0,0 +1,21 @@
+namespace DigitalData.MessagingService.Domain.Exceptions
+{
+    /// <summary>
+    /// Exception thrown when a requested entity is not found.
+    /// </summary>
+    public class NotFoundException : Exception
+    {
+        public NotFoundException(string entityName, object key) : base($"{entityName} with key '{key}' was not found.")
+        {
+        }
+
+        public NotFoundException(string message) : base(message)
+        {
+        }
+
+        public NotFoundException(string message, Exception innerException)
+            : base(message, innerException)
+        {
+        }
+    }
+}