Chore: Move identity and errutil to apimachinery module (#89116)

pkg/apimachinery/errutil/source.go

@@ -0,0 +1,18 @@
+package errutil
+
+// Source identifies from where an error originates.
+type Source string
+
+const (
+	// SourceServer implies error originates from within the server, i.e. this application.
+	SourceServer Source = "server"
+
+	// SourceDownstream implies error originates from response error while server acting
+	// as a proxy, i.e. from a downstream service.
+	SourceDownstream Source = "downstream"
+)
+
+// IsDownstream checks if Source is SourceDownstream.
+func (s Source) IsDownstream() bool {
+	return s == SourceDownstream
+}