chore: move to external-dns's domain filter interface

This had me confused for the last couple days. I don't understand why we don't give the actual DomainFilter here.

internal/unifi/provider.go

@@ -17,7 +17,7 @@ type Provider struct {
 	domainFilter endpoint.DomainFilter
 }
 
-// newUnifiProvider initializes a new DNSProvider.
+// NewUnifiProvider initializes a new DNSProvider.
 func NewUnifiProvider(domainFilter endpoint.DomainFilter, config *Config) (provider.Provider, error) {
 	c, err := newUnifiClient(config)
 
@@ -77,6 +77,6 @@ func (p *Provider) ApplyChanges(ctx context.Context, changes *plan.Changes) erro
 }
 
 // GetDomainFilter returns the domain filter for the provider.
-func (p *Provider) GetDomainFilter() endpoint.DomainFilter {
+func (p *Provider) GetDomainFilter() endpoint.DomainFilterInterface {
 	return p.domainFilter
 }

pkg/webhook/webhook.go

@@ -200,19 +200,13 @@ func (p *Webhook) Negotiate(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	b, err := p.provider.GetDomainFilter().MarshalJSON()
+	w.Header().Set(contentTypeHeader, string(mediaTypeVersion1))
+	err := json.NewEncoder(w).Encode(p.provider.GetDomainFilter())
 	if err != nil {
 		requestLog(r).Error("failed to marshal domain filter")
 		w.WriteHeader(http.StatusInternalServerError)
 		return
 	}
-
-	w.Header().Set(contentTypeHeader, string(mediaTypeVersion1))
-	if _, writeError := w.Write(b); writeError != nil {
-		requestLog(r).With(zap.Error(writeError)).Error("error writing response")
-		w.WriteHeader(http.StatusInternalServerError)
-		return
-	}
 }
 
 func requestLog(r *http.Request) *zap.Logger {