feat: Implement structured MQTT messages with separate username and message

This commit refactors the chat application to transmit MQTT messages using a structured JSON format.
The  struct, leveraging , now encapsulates both the username and the message content, allowing them to be sent and received as distinct fields.

Key changes include:
- Added  and  dependencies to .
- Defined a  struct with  and  fields.
- Modified message publishing to serialize  instances to JSON.
- Updated message receiving to deserialize JSON payloads into  instances.
- Adjusted internal message storage and display logic to handle the separate username and message components.
- Ensured the connect message also uses the new structured format.

Cargo.lock

@@ -617,6 +617,38 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "serde"
+version = "1.0.219"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5f0e2c6ed6606019b4e29e69dbaba95b11854410e5347d525002456dbbb786b6"
+dependencies = [
+ "serde_derive",
+]
+
+[[package]]
+name = "serde_derive"
+version = "1.0.219"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5b0276cf7f2c73365f7157c8123c21cd9a50fbbd844757af28ca1f5925fc2a00"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
+[[package]]
+name = "serde_json"
+version = "1.0.140"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "20068b6e96dc6c9bd23e01df8827e6c7e1f2fddd43c21810382803c136b99373"
+dependencies = [
+ "itoa",
+ "memchr",
+ "ryu",
+ "serde",
+]
+
 [[package]]
 name = "shlex"
 version = "1.3.0"
@@ -820,6 +852,8 @@ dependencies = [
  "rand",
  "ratatui",
  "rumqttc",
+ "serde",
+ "serde_json",
  "tokio",
  "tokio-stream",
 ]