[Java SDK] Implement pluggable serialization framework

**Title:** `[Java SDK] Implement pluggable serialization framework`

**Description:**
Current Message class only supports basic byte[] and String, requiring users to implement their own serialization.

**Proposed Solution:**
```java
// Pluggable serializers
public interface MessageSerializer {
byte[] serialize(T data);
Map headers(T data);
}

public interface MessageDeserializer {
T deserialize(byte[] data, Map headers);
}

// Built-in serializers
- JsonSerializer/Deserializer
- AvroSerializer/Deserializer
- ProtobufSerializer/Deserializer
```