Add jgvariant-ostree module.

Change-Id: Idf7bacad28d7cf65eb1ddd0994dcc2c2c2a7e18e
diff --git a/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/SummarySignature.java b/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/SummarySignature.java
new file mode 100644
index 0000000..a05b96d
--- /dev/null
+++ b/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/SummarySignature.java
@@ -0,0 +1,35 @@
+package eu.mulk.jgvariant.ostree;
+
+import eu.mulk.jgvariant.core.Decoder;
+import eu.mulk.jgvariant.core.Variant;
+import java.nio.charset.StandardCharsets;
+import java.util.List;
+
+/**
+ * A collection of cryptographic signatures for a {@link Summary}.
+ *
+ * <p>Stored as a file named {@code summary.sig} in the OSTree repository root.
+ *
+ * <p>Reference: {@code ostree-repo-static-delta-private.h#OSTREE_SUMMARY_SIG_GVARIANT_STRING}
+ */
+public record SummarySignature(List<Signature> signatures) {
+
+  /** A cryptographic signature. */
+  public record Signature(String key, Variant data) {
+
+    private static final Decoder<Signature> DECODER =
+        Decoder.ofStructure(
+            Signature.class, Decoder.ofString(StandardCharsets.UTF_8), Decoder.ofVariant());
+
+    public static Decoder<Signature> decoder() {
+      return DECODER;
+    }
+  }
+
+  private static final Decoder<SummarySignature> DECODER =
+      Decoder.ofArray(Signature.decoder()).map(SummarySignature::new);
+
+  public static Decoder<SummarySignature> decoder() {
+    return DECODER;
+  }
+}