Add list endpoint to health service

Motivation:
With gRFC A90 a `List` endpoint was added to the health service.

Modifications:
* Update the health.proto file to the new version
* Update the generated files using protoc
* Add the new `List` endpoint to the HealthService
* Add testing for the list endpoint

Result:
HealthService is now fully compliant with gRFC A90.

Author: yigityazicilar