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_test.go

@@ -89,3 +89,33 @@ gCDlEzkuOCybCHf+q766bve799L7Y5y5oRsHY1MrUCUwYF/tL7Sg7EYMsA==
 		})
 	}
 }
+
+func TestIsBase64(t *testing.T) {
+	tests := map[string]struct {
+		signature []byte
+		wantErr   bool
+	}{
+		"valid base64": {
+			signature: []byte("SGVsbG8gV29ybGQ="),
+			wantErr:   false,
+		},
+		"invalid base64": {
+			signature: []byte("not base64"),
+			wantErr:   true,
+		},
+		"empty input": {
+			signature: []byte{},
+			wantErr:   false,
+		},
+	}
+
+	for tc, tt := range tests {
+		t.Run(tc, func(t *testing.T) {
+			err := IsBase64(tt.signature)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("IsBase64() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+		})
+	}
+}