refactor(v2): init events (#7823)

creates events structures for initial projections and read models

internal/v2/instance/aggregate.go

@@ -0,0 +1,8 @@
+package instance
+
+import "github.com/zitadel/zitadel/internal/repository/instance"
+
+const (
+	AggregateType   = string(instance.AggregateType)
+	eventTypePrefix = AggregateType + "."
+)