feat: Enhance A2A integration with modular architecture

Address review comments from João:
- Add TaskInfo dataclass for enhanced task management with status tracking
- Add ServerConfig dataclass for improved server configuration management
- Add custom exception classes (A2AServerError, TransportError, ExecutionError)
- Refactor code to use modular components for better maintainability
- Update output conversion to handle JSON data types properly
- Improve error handling with granular exception types
- All tests pass (30 passed, 6 skipped)

Co-Authored-By: João <joao@crewai.com>

src/crewai/a2a/exceptions.py

@@ -0,0 +1,16 @@
+"""Custom exceptions for A2A integration."""
+
+
+class A2AServerError(Exception):
+    """Base exception for A2A server errors."""
+    pass
+
+
+class TransportError(A2AServerError):
+    """Error related to transport configuration."""
+    pass
+
+
+class ExecutionError(A2AServerError):
+    """Error during crew execution."""
+    pass