Add support for CoreDNS alternate plugin to have fallback DNS servers in case a Disaster

1. Proposed title of this feature request
Add support for CoreDNS alternate plugin to have fallback DNS servers in case a Disaster

2. What is the nature and description of the request?
To cover potential Disaster scenarios where DNS at the customer site becomes unavailable but a potential 2nd or 3rd location might have backup DNS servers available, it would be helpful to have the CoreDNS alternate plugin (https://coredns.io/explugins/alternate/) available and possibility to configure it so that OpenShift Container Platform 4 can fallback to such Backup DNS infrastructure in case the primary upstream servers defined timeout or reporting failures (SERVFAIL)

3. Why does the customer need this? (List the business requirements here)
During major faults in DNS infrastructure or connectivtiy to such infrastructure, it would be helpful to have the possibility to configure alternate DNS servers using https://coredns.io/explugins/alternate/ for upstream DNS servers that are being used when the primary defined servers are failing to respond to queries (SERVFAIL). Certainly such backup infrastructure would need to be available but in all major disaster recovery plans something like that is available. So instead of requiring to reconfigure the entire OpenShift Container Platform 4 - Cluster to point to those backup DNS servers, it would be nice to have the configured right away and have CoreDNS failing over automatically when SERVFAIL is reported because of primary DNS infrastucture being unavailable.

4. List any affected packages or components.
DNS Cluster Operator and CoreDNS