Implement health Endpoint for Health Check in PowerPay Server

[stephane-segning]

• Description: Using a backend, we want to know if the app is running or not. Having a /health endpoint is common. In Spring Boot, we have something called actuator that implement these features.
• Implementation:
  • Add the actuator dependency in the backend project
  • Expose all metrics to the actuator
  • Test the APIs to ensure it's working
  • Write unit testing for the said endpoint

[stephane-segning]

Done in #75 & #79. Thank you @Awambeng @chendiblessing @mbunwe-victor