Add strict values.schema.json and the ingress gate to patchmon

Two changes of the same class - values that do not do what they promise:

1. values.schema.json (#68): additionalProperties: false on every
   chart-owned object level, so a key the chart does not read is rejected
   at render time instead of being silently ignored. Objects passed
   verbatim into the pod spec (probes, securityContext, global) stay open.

2. Ingress gate (#93): the Ingress template is wrapped in
   {{- if .Values.ingress.enabled }} and the key is added to values.yaml
   with default true, so behaviour is unchanged. Without the gate the
   value did not exist at all and could not switch the Ingress off.

Major bump: a schema can break existing deployments that carry dead keys.

patchmon/values.yaml

@@ -136,6 +136,10 @@ patchmon:
     env: []
 
 ingress:
+  # Set to false to omit the Ingress entirely (the required domain and
+  # clusterIssuer values are then not needed either, because they are only
+  # read inside the Ingress template).
+  enabled: true
   clusterIssuer: null
   domain: null
   className: nginx