feat(packbuilder): add bindings and api

lib/src/packbuilder.dart

@@ -0,0 +1,73 @@
+import 'dart:ffi';
+import 'package:libgit2dart/libgit2dart.dart';
+import 'bindings/libgit2_bindings.dart';
+import 'bindings/packbuilder.dart' as bindings;
+
+class PackBuilder {
+  /// Initializes a new instance of [PackBuilder] class.
+  ///
+  /// Should be freed with `free()`.
+  ///
+  /// Throws a [LibGit2Error] if error occured.
+  PackBuilder(Repository repo) {
+    _packbuilderPointer = bindings.init(repo.pointer);
+  }
+
+  late final Pointer<git_packbuilder> _packbuilderPointer;
+
+  /// Pointer to memory address for allocated packbuilder object.
+  Pointer<git_packbuilder> get pointer => _packbuilderPointer;
+
+  /// Adds a single object.
+  ///
+  /// For an optimal pack it's mandatory to add objects in recency order,
+  /// commits followed by trees and blobs.
+  ///
+  /// Throws a [LibGit2Error] if error occured.
+  void add(Oid oid) {
+    bindings.add(
+      packbuilderPointer: _packbuilderPointer,
+      oidPointer: oid.pointer,
+    );
+  }
+
+  /// Recursively adds an object and its referenced objects.
+  ///
+  /// Adds the object as well as any object it references.
+  ///
+  /// Throws a [LibGit2Error] if error occured.
+  void addRecursively(Oid oid) {
+    bindings.addRecursively(
+      packbuilderPointer: _packbuilderPointer,
+      oidPointer: oid.pointer,
+    );
+  }
+
+  /// Writes the new pack and corresponding index file to [path] if provided
+  /// or default location.
+  ///
+  /// Throws a [LibGit2Error] if error occured.
+  void write(String? path) {
+    bindings.write(packbuilderPointer: _packbuilderPointer, path: path);
+  }
+
+  /// Returns the total number of objects the packbuilder will write out.
+  int get length => bindings.length(_packbuilderPointer);
+
+  /// Returns the number of objects the packbuilder has already written out.
+  int get writtenLength => bindings.writtenCount(_packbuilderPointer);
+
+  /// Sets and returns the number of threads to spawn.
+  ///
+  /// By default, libgit2 won't spawn any threads at all; when set to 0,
+  /// libgit2 will autodetect the number of CPUs.
+  int setThreads(int number) {
+    return bindings.setThreads(
+      packbuilderPointer: _packbuilderPointer,
+      number: number,
+    );
+  }
+
+  /// Releases memory allocated for packbuilder object.
+  void free() => bindings.free(_packbuilderPointer);
+}