feat(attr): add bindings and api

lib/src/bindings/attr.dart

@@ -0,0 +1,44 @@
+import 'dart:ffi';
+import 'package:ffi/ffi.dart';
+import '../error.dart';
+import '../util.dart';
+import 'libgit2_bindings.dart';
+
+/// Look up the value of one git attribute for path.
+///
+/// Returned value can be either `true`, `false`, `null` (if the attribute was not set at all),
+/// or a [String] value, if the attribute was set to an actual string.
+///
+/// Throws a [LibGit2Error] if error occured.
+dynamic getAttribute({
+  required Pointer<git_repository> repoPointer,
+  required int flags,
+  required String path,
+  required String name,
+}) {
+  final out = calloc<Pointer<Int8>>();
+  final pathC = path.toNativeUtf8().cast<Int8>();
+  final nameC = name.toNativeUtf8().cast<Int8>();
+  final error = libgit2.git_attr_get(out, repoPointer, flags, pathC, nameC);
+
+  calloc.free(pathC);
+  calloc.free(nameC);
+
+  if (error < 0) {
+    throw LibGit2Error(libgit2.git_error_last());
+  }
+
+  final attributeValue = libgit2.git_attr_value(out.value);
+
+  if (attributeValue == git_attr_value_t.GIT_ATTR_VALUE_UNSPECIFIED) {
+    return null;
+  } else if (attributeValue == git_attr_value_t.GIT_ATTR_VALUE_TRUE) {
+    return true;
+  } else if (attributeValue == git_attr_value_t.GIT_ATTR_VALUE_FALSE) {
+    return false;
+  } else if (attributeValue == git_attr_value_t.GIT_ATTR_VALUE_STRING) {
+    return out.value.cast<Utf8>().toDartString();
+  } else {
+    throw Exception('The attribute value from libgit2 is invalid');
+  }
+}