move nostr-sdk repository into here because why not?

sdk/helpers.go

@@ -0,0 +1,21 @@
+package sdk
+
+import (
+	"strings"
+)
+
+// IsVirtualRelay returns true if the given normalized relay URL shouldn't be considered for outbox-model calculations.
+func IsVirtualRelay(url string) bool {
+	if len(url) < 6 {
+		// this is just invalid
+		return true
+	}
+
+	if strings.HasPrefix(url, "wss://feeds.nostr.band") ||
+		strings.HasPrefix(url, "wss://filter.nostr.wine") ||
+		strings.HasPrefix(url, "wss://cache") {
+		return true
+	}
+
+	return false
+}