Employee Module | Update Employee

- [x] Employee Model
- [x] Employee Validator - update employee
- [x] Employee DAO - update employee
- [x] Employee Service - update employee

## Issue Title: Implement "Update Employee" Functionality in Employee Module

### Issue Description

We need to implement the "Update Employee" functionality within the Employee Module. This involves updating the Employee Model, enhancing the Employee Validator for updating employees, and implementing the necessary methods in the Employee DAO and Employee Service.

### Task Breakdown

1. **Employee Model:**
- Review and update the Employee Model class to accommodate any changes required for updating employee information.
- Ensure that the model attributes align with the data that needs to be updated, such as name, contact information, etc.

2. **Employee Validator:**
- Enhance the Employee Validator to include validation rules for updating employee information.
- Validate that the input data for updating employees adheres to the required criteria, such as non-empty name, valid contact details, etc.
- Provide informative error messages for invalid update requests.

3. **Employee DAO:**
- Implement the "updateEmployee" method in the Employee DAO class.
- This method should take an Employee object or employee ID as a parameter and update the corresponding employee's information in the database or data store.
- Handle any potential exceptions gracefully and provide clear error messages.

4. **Employee Service:**
- Create an "updateEmployee" service method in the Employee Service class.
- This method should serve as an intermediary between the Validator and DAO.
- Validate the input data for updating an employee using the Employee Validator.
- If the data is valid, call the "updateEmployee" method in the Employee DAO to update the employee's information.
- Return appropriate responses to indicate success or failure.

### Additional Information

Please consider the following guidelines while working on this issue:
- Follow the coding conventions and best practices of the project.
- Ensure that unit tests are written and that the update functionality is thoroughly tested.
- Document any changes made to the codebase.
- Create meaningful commit messages that reference this issue for better traceability.