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/presentation/DigitalData.MessagingService.API/appsettings.Secrets.json

@@ -0,0 +1,27 @@
+{
+  "AllowedHosts": "*",
+  "RabbitMQ": {
+    "HostName": "172.24.12.56",
+    "Port": 5672,
+    "UserName": "admin",
+    "Password": "fl!'D}4;pYBb\\VD&{6]]G*\\0Bq8fVIn0j?Sgm\\2A,6GS47g5Dj",
+    "VirtualHost": "/",
+    "AutomaticRecoveryEnabled": true,
+    "NetworkRecoveryIntervalSeconds": 10,
+    "QueueName": "emailprofiler.email.outbox",
+    "ExchangeName": "emailprofiler.emails",
+    "RoutingKey": "email.outbox",
+    "DlqQueueName": "emailprofiler.email.outbox.dlq",
+    "DlqExchangeName": "emailprofiler.emails.dlq",
+    "DlqRoutingKey": "email.outbox.dlq"
+  },
+  "EmailAccount": {
+    "Username": "test-flow@digitaldata.works",
+    "Password": "ddemail108",
+    "PasswordEncrypted": false,
+    "SmtpServer": "kundencenter.triplew.de",
+    "SmtpPort": 465,
+    "SmtpUseSsl": true,
+    "UseOAuth2": false
+  }
+}