feat!: remove MockTUFClient (#135)

* feat! remove MockTUFClient

*Breaking*
- use LocalPolicyDir and nil TUFClient instead

Other:
- add stateful Verifier

pkg/policy/types.go

@@ -28,6 +28,7 @@ type Result struct {
 
 type Options struct {
 	TUFClientOptions *tuf.ClientOptions
+	DisableTUF       bool
 	LocalTargetsDir  string
 	LocalPolicyDir   string
 	PolicyID         string