Add a health endpoint

[alexgarel]

What

• It's useful to probe the service and generate alerts.

Code pointers

• https://robotoff.openfoodfacts.org/api/v1/health

Part of

• 🎯 What can I work on ? (search-a-licious) #62

[Marc-AntoineA]

Some notes before starting dev:

• robotoff/health.py code : https://github.com/openfoodfacts/robotoff/blob/main/robotoff/health.py
• Dependency : https://pypi.org/project/py-healthcheck/
• How to use Health in api.py : https://github.com/openfoodfacts/robotoff/blob/ef962195e66236a401356573595b0861186394e0/robotoff/app/api.py#L1549