feat: Implement message encryption and decryption support

- Added IMessageEncryptionService and its implementation MessageEncryptionService for handling message encryption.
- Updated ChannelsController and ChatService to encrypt messages before storing and sending.
- Introduced encryption key retrieval endpoint in ServerController.
- Modified EchoHubDbContext to accommodate increased message content and embed JSON lengths for encrypted data.
- Created migrations to support encryption-related database changes.
- Enhanced FirstRunSetup to ensure encryption key is generated if not present.
- Updated appsettings.example.json to include encryption configuration.
- Added comprehensive unit tests for encryption service and compatibility tests between client and server encryption.

src/EchoHub.Server/appsettings.example.json

@@ -14,6 +14,10 @@
     "PublicServer": false,
     "PublicHost": ""
   },
+  "Encryption": {
+    "Key": "",
+    "EncryptDatabase": false
+  },
   "Irc": {
     "Enabled": false,
     "Port": 6667,