With the introduction of CiliumClusterwideNetworkPolicies a policy has
the ability to select the 'reserved:health' endpoints. If a user applies
a policy such as [0], it will block all connectivity for the health
endpoints. As this does not seem expected, since Cilium will report that
the connectivity across Cilium pods is not working, the user will need to
deploy a CCNP that allows connectivity between health endpoints. This
can be done with [1].

This commit introduces 'reserved:health' as an entity that can be
selected by CNP and / or CCNP.

[0]
```
---
apiVersion: cilium.io/v2
kind: CiliumClusterwideNetworkPolicy
metadata:
  name: ingress-deny-all
spec:
  endpointSelector:
    matchLabels: {}
  ingress:
  - {}
---
apiVersion: cilium.io/v2
kind: CiliumClusterwideNetworkPolicy
metadata:
  name: egress-deny-all
spec:
  endpointSelector:
    matchLabels: {}
  egress:
  - {}
```

[1]
```
apiVersion: "cilium.io/v2"
kind: CiliumClusterwideNetworkPolicy
metadata:
  name: "allow-reserved-health"
spec:
  endpointSelector:
    matchLabels:
      'reserved:health': ""
  ingress:
    - fromEntities:
      - health
```

Signed-off-by: André Martins <andre@cilium.io>