Handle errors from Go in Rego. Support for skipping TL (#47)

* Make TL logging/verification optional

* Return errors from go-lang fns

* Update pkg/policy/rego.go

Co-authored-by: Jonny Stoten <jonny@jonnystoten.com>

* Update pkg/attestation/sign.go

Co-authored-by: Joel Kamp <joel.kamp@docker.com>

* Move public key marshelling until later

* Simplify logSignature and pass down opts

---------

Co-authored-by: Jonny Stoten <jonny@jonnystoten.com>
Co-authored-by: Joel Kamp <joel.kamp@docker.com>

pkg/attestation/verify_test.go

@@ -39,8 +39,11 @@ func TestVerifyUnsignedAttestation(t *testing.T) {
 		Payload:     base64.StdEncoding.EncodeToString(payload),
 		PayloadType: intoto.PayloadType,
 	}
+	opts := &attestation.VerifyOptions{
+		Keys: attestation.Keys{},
+	}
 
-	_, err := attestation.VerifyDSSE(ctx, env, attestation.KeysMap{})
+	_, err := attestation.VerifyDSSE(ctx, env, opts)
 	assert.Error(t, err)
 	assert.Contains(t, err.Error(), "no signatures")
 }