LibELF+LibCoreDump: Add a Metadata notes entry

This is a new NotesEntry type which will allow applications to embed
arbitrary metadata in crashdumps (stored as a JSON string). It will be
used to store an assertion message, for example.

Libraries/LibELF/CoreDump.h

@@ -39,6 +39,7 @@ struct [[gnu::packed]] NotesEntryHeader
         ProcessInfo,
         ThreadInfo,
         MemoryRegionInfo,
+        Metadata,
     };
     Type type;
 };
@@ -82,4 +83,10 @@ struct [[gnu::packed]] MemoryRegionInfo
     }
 };
 
+struct [[gnu::packed]] Metadata
+{
+    NotesEntryHeader header;
+    char json_data[]; // Null terminated
+};
+
 }