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>

test/testdata/local-policy-fail/doi/policy.rego

@@ -12,14 +12,24 @@ keys := [{
 	"signing-format": "dssev1",
 }]
 
+provs(pred) := p if {
+	res := attest.fetch(pred)
+	not res.error
+	p := res.value
+}
+
 atts := union({
-	attestations.attestation("https://slsa.dev/provenance/v0.2"),
-	attestations.attestation("https://spdx.dev/Document"),
+	provs("https://slsa.dev/provenance/v0.2"),
+	provs("https://spdx.dev/Document"),
 })
 
+opts := {"keys": keys}
+
 statements contains s if {
 	some att in atts
-	s := attestations.verify_envelope(att, keys)
+	res := attest.verify(att, opts)
+	not res.error
+	s := res.value
 }
 
 subjects contains subject if {