feat: support code generation

[simonpham]

Description

As a developer, I expect to only define my keys as an Enum class, then build_runner will help me to generate the required code. For example:

settings.dart

part 'settings.g.dart';

@EasyBoxClass(
  isLazy: false,
  isEncrypted: true,
  boxKey: 'settings',
  generatedClassName: 'SettingsBox',
)
enum Settings {
  @EasyBoxField(ThemeMode, ThemeMode.system)
  themeMode,
  @EasyBoxField(int, 0)
  counter,
}

then after run flutter pub run build_runner build, I got the full implementation of SettingsBox as following easy_hive documentation.