feat(tag): add bindings and api

lib/src/tag.dart

@@ -0,0 +1,94 @@
+import 'dart:ffi';
+
+import 'bindings/libgit2_bindings.dart';
+import 'bindings/tag.dart' as bindings;
+import 'bindings/object.dart' as object_bindings;
+import 'commit.dart';
+import 'oid.dart';
+import 'repository.dart';
+import 'signature.dart';
+import 'enums.dart';
+
+class Tag {
+  /// Initializes a new instance of [Tag] class from provided
+  /// [Repository] and [Oid] objects.
+  ///
+  /// Should be freed with `free()` to release allocated memory.
+  Tag.lookup(Repository repo, Oid oid) {
+    _tagPointer = bindings.lookup(repo.pointer, oid.pointer);
+  }
+
+  late final Pointer<git_tag> _tagPointer;
+
+  /// Pointer to memory address for allocated tag object.
+  Pointer<git_tag> get pointer => _tagPointer;
+
+  /// Creates a new tag in the repository from provided Oid object.
+  ///
+  /// A new reference will also be created pointing to this tag object. If force is true
+  /// and a reference already exists with the given name, it'll be replaced.
+  ///
+  /// The message will not be cleaned up.
+  ///
+  /// The tag name will be checked for validity. You must avoid the characters
+  /// '~', '^', ':', '\', '?', '[', and '*', and the sequences ".." and "@{" which have
+  /// special meaning to revparse.
+  ///
+  /// Throws a [LibGit2Error] if error occured.
+  static Oid create({
+    required Repository repository,
+    required String tagName,
+    required Oid target,
+    required GitObject targetType,
+    required Signature tagger,
+    required String message,
+    bool force = false,
+  }) {
+    // add 1 to GitObject enum index to match libgit2
+    final object = object_bindings.lookup(
+      repository.pointer,
+      target.pointer,
+      targetType.index + 1,
+    );
+    final result = bindings.create(
+      repository.pointer,
+      tagName,
+      object,
+      tagger.pointer,
+      message,
+      force,
+    );
+
+    object_bindings.free(object);
+    return Oid(result);
+  }
+
+  /// Get the tagged object of a tag.
+  ///
+  /// This method performs a repository lookup for the given object and returns it.
+  ///
+  /// Throws a [LibGit2Error] if error occured.
+  Commit get target => Commit(bindings.target(_tagPointer).cast());
+
+  /// Get the id of a tag.
+  Oid get id => Oid(bindings.id(_tagPointer));
+
+  /// Returns the name of a tag.
+  String get name => bindings.name(_tagPointer);
+
+  /// Returns the message of a tag.
+  String get message => bindings.message(_tagPointer);
+
+  /// Returns the tagger (author) of a tag if there is one.
+  Signature? get tagger {
+    final sigPointer = bindings.tagger(_tagPointer);
+    if (sigPointer != nullptr) {
+      return Signature(sigPointer);
+    } else {
+      return null;
+    }
+  }
+
+  /// Releases memory allocated for tag object.
+  void free() => bindings.free(_tagPointer);
+}