This commit improves the `health` endpoint to run health check for a
particular component or, if that component is itself a composite, an
instance of that component.

Concretely, it is now possible to issue a `GET` on
`/actuator/health/{component}` and
`/actuator/health/{component}/instance` to retrieve the health of a
component or an instance of a composite component, respectively.

If details cannot be showed for the current user, any request leads to a
404 and does not invoke the health check at all.

Closes gh-8865