api: refine signature types

Wrapping apiObject does not work as intended as the version field
is when fetching objects from the API. Thus we need to insert
the target path of the signature directly.

internal/sigstore/verify.go

@@ -63,3 +63,10 @@ func (c CosignVerifier) VerifySignature(content, signature []byte) error {
 
 	return nil
 }
+
+// IsBase64 checks if the given byte slice is base64 encoded.
+func IsBase64(signature []byte) error {
+	target := make([]byte, base64.StdEncoding.DecodedLen(len(signature)))
+	_, err := base64.StdEncoding.Decode(target, signature)
+	return err
+}