feat(reference): add ability to create direct reference

lib/src/odb.dart

@@ -0,0 +1,31 @@
+import 'dart:ffi';
+import 'bindings/libgit2_bindings.dart';
+import 'bindings/odb.dart' as bindings;
+import 'util.dart';
+
+class Odb {
+  /// Initializes a new instance of [Odb] class from provided
+  /// pointer to Odb object in memory.
+  Odb(this._odbPointer) {
+    libgit2.git_libgit2_init();
+  }
+
+  /// Pointer to memory address for allocated oid object.
+  late final Pointer<git_odb> _odbPointer;
+
+  /// Determine if an object can be found in the object database by an abbreviated object ID.
+  ///
+  /// Throws a [LibGit2Error] if error occured.
+  Pointer<git_oid> existsPrefix(
+    Pointer<git_oid> shortOid,
+    int len,
+  ) {
+    return bindings.existsPrefix(_odbPointer, shortOid, len);
+  }
+
+  /// Releases memory allocated for odb object.
+  void free() {
+    bindings.free(_odbPointer);
+    libgit2.git_libgit2_shutdown();
+  }
+}