Should provide GZ_ADD_PLUGIN as an alias to IGNITION_ADD_PLUGIN and friends

[azeey]

Desired behavior

It would be convenient to have GZ_ADD_PLUGIN in ign-plugin1 so we'd have less things to change when porting from Fortress to Garden

Alternatives considered

n/a

Implementation suggestion

Add aliases to Register.hh

#define GZ_ADD_PLUGIN(PluginClass, ...) \
  DETAIL_IGNITION_ADD_PLUGIN(PluginClass, __VA_ARGS__)

#define GZ_ADD_PLUGIN_ALIAS(PluginClass, ...) \
  DETAIL_IGNITION_ADD_PLUGIN_ALIAS(PluginClass, __VA_ARGS__)

#define GZ_ADD_FACTORY(ProductType, FactoryType) \
  DETAIL_IGNITION_ADD_FACTORY(ProductType, FactoryType)

#define GZ_ADD_FACTORY_ALIAS(ProductType, FactoryType, ...) \
  DETAIL_IGNITION_ADD_FACTORY_ALIAS(ProductType, FactoryType, __VA_ARGS__)