ROX-34488: verify sigstore bundle signatures via sigstore-go

Add verifySigstoreBundle which verifies DSSE-envelope signatures using
sigstore-go directly, bypassing the cosign SimpleSigning path. The
verifier now branches on signature format: SimpleSigning signatures
continue through cosign.VerifyImageSignature, while sigstore bundles
use sigstore-go's verify.SignedEntity with the same trust roots
(public key or certificate identity/issuer).

Partially generated by AI.

Author: stehessel

go.mod

@@ -121,6 +121,7 @@ require (
 	github.com/sigstore/cosign/v3 v3.1.2
 	github.com/sigstore/rekor v1.5.3
 	github.com/sigstore/sigstore v1.10.8
+	github.com/sigstore/sigstore-go v1.2.1
 	github.com/sourcegraph/conc v0.3.1-0.20240121214520-5f936abd7ae8
 	github.com/spf13/cobra v1.10.2
 	github.com/spf13/pflag v1.0.10
@@ -405,7 +406,6 @@ require (
 	github.com/shopspring/decimal v1.4.0 // indirect
 	github.com/sigstore/protobuf-specs v0.5.1 // indirect
 	github.com/sigstore/rekor-tiles/v2 v2.3.0 // indirect
-	github.com/sigstore/sigstore-go v1.2.1 // indirect
 	github.com/sigstore/timestamp-authority/v2 v2.1.2 // indirect
 	github.com/sirupsen/logrus v1.9.4 // indirect
 	github.com/skeema/knownhosts v1.3.1 // indirect