When the --events flag is passed at startup, Source.AddEventHandler() is called
on each configured source. Most sources provide AddEventHandler()
implementations that invoke the reconciliation loop when the configured source
changes, but the CRD source had a no-op implementation. I.e. when a custom
resource was created, updated, or deleted, external-dns remained unware, and the
reconciliation loop would not fire until the configured interval had passed.
This change adds an informer (on the CRD specified by --crd-source-apiversion
and --crd-source-kind=DNSEndpoint), and a Source.AddEventHandler()
implementation that calls Informer.AddEventHandler(). Now when a custom
resource is created, updated, or deleted, the reconciliation loop is invoked.
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