As a platform engineer, I would like to configure a default DNS provider for a given namespace rather than having to specify a reference to a specific DNS provider for every DNS policy.

What

Allow a DNS provider secret to be created in a namespace that is then marked as the default. There can only be one default in a given namesapce. Any policy created without a specific reference to a DNS Provider credential will instead fall back to use the default DNS provider if one is configured. 

Acceptance Criterea (examples)

• I can create a DNSProvider credential in a namespace and mark it as a default
• I can clearly see from a DNSPolicy when it is using a default provider
• If I create multiple defaults in a namespace, I should see this as a reason why the DNSPolicy is not accepted