Add service class interface with helper methods

[ben-ryder]

Package Information: core

Describe your feature
Add a base service class interface which has the methods async onStart() and async onKill(). These are like "hook functions" which KangoJS can call at certain points of the application lifecycle.

• onStart - called once the application is setup and ready. This could be used for things like starting up cron tasks etc
• onKill - called when the application is being killed. This can be used for clean up such as closing connections to the database/Redis etc

Is your suggestion related to any problems?
For testing I need to manually fetch all services that require clean up and handle each one.
Having a built in way to handle this would make it easier when multiple services need clean up.

Additional context
n/a