rotate: narrow a pair credential to one consuming module (hq issue 268)

A machine runs many consumers of one provision, each with its own
credential. When one module leaks its credential, `rotate <provision>
--consumer <machine>` was the narrowest act and replaced every module's
on that machine, restarting all of them. --module (and the verb's
module argument beside provision) rotates only that module's.

cmd/mesh-controller/seatverbs_test.go

@@ -43,6 +43,10 @@ func TestRotateTakesAProvisionOrAnOwnSecret(t *testing.T) {
 	if strings.Join(argv, " ") != "rotate postgres-database --consumer ace" {
 		t.Fatalf("a pair credential: %v", argv)
 	}
+	argv, _ = argvFor("rotate", map[string]any{"provision": "postgres-database", "consumer": "ace", "module": "letta"})
+	if strings.Join(argv, " ") != "rotate postgres-database --consumer ace --module letta" {
+		t.Fatalf("one consuming module's pair credential: %v", argv)
+	}
 	argv, _ = argvFor("rotate", map[string]any{"node": "ace", "module": "nodered", "secret": "api-token"})
 	if strings.Join(argv, " ") != "secret rotate ace nodered api-token" {
 		t.Fatalf("an own secret: %v", argv)