auth: Trust EDNS Client Subnet from a trusted notification proxy

auth: Trust EDNS Client Subnet from a trusted notification proxy

This allows for example the use of dnsdist in front of supermaster
slaves.
dnsdist must be configured to send ECS to the backend with:
* `useClientSubnet=true` on the corresponding `newServer()`
* `setECSSourcePrefixV4(32)` and/or `setECSSourcePrefixV6(128)` so
the exact source is sent to the slave
* `setECSOverride(true)` so that any existing ECS information is
overridden

In addition, pdns must be configured to accept notification from
dnsdist with `trusted-notification-proxy` and to process ECS with
`edns-subnet-processing=yes`.