Provide health check API without certs required when TLS enabled

## Feature Request

### Describe your feature request related problem

When we deploy PD in the platforms like Kubernetes, we can leverage the health check mechanism provided by Kubernetes to exclude the unhealthy Pod from the service endpoint automatically, to do that, we need to provide a health check API.
We have the API `/pd/api/v1/health`, however, when TLS is enabled, certs are required to access this API, which is unavailable for Kubelet, so we need to provide a health check API without certs required when TLS enabled.

### Describe the feature you'd like

### Describe alternatives you've considered

### Teachability, Documentation, Adoption, Migration Strategy