Prepare multiple physics engines support

[diegoferigo]

The current logic of the vendored Physics system allows loading the ign-physics plugin, among other options, by reading the PhysicsEnginePlugin component. Until now, we just supported DART (the only available "real" physics engine) and we were relying on the fact that it loads it by default.

This PR updates our logic to use the PhysicsEnginePlugin component. This is in preparation of the upcoming bullet plugin.

---

Note that we do not currently support loading custom ignition physics plugin. This support would mean passing the OS-agnostic name of the plugin, and without introducing new methods, adding a new argument would break APIs (even though, only the ScenarIO Gazebo API, and not the ScenarIO core APIs).

Since there are no third-party plugins yet, let's leave this option to a future PR. If there's interest from downstream, adding a new method would be trivial.

gym-ignition/cpp/scenario/plugins/Physics/Physics.cc

Lines 492 to 521 in 9394833

	// 1. Engine from component (from command line / ServerConfig)
	auto engineComp = _ecm.Component<components::PhysicsEnginePlugin>(_entity);
	if (engineComp && !engineComp->Data().empty())
	{
	pluginLib = engineComp->Data();
	}
	// 2. Engine from SDF
	else if (_sdf->HasElement("engine"))
	{
	auto sdfClone = _sdf->Clone();
	auto engineElem = sdfClone->GetElement("engine");
	pluginLib = engineElem->Get<std::string>("filename", pluginLib).first;
	}
	// 3. Use DART by default
	if (pluginLib.empty())
	{
	pluginLib = "libignition-physics-dartsim-plugin.so";
	}
	// Update component
	if (!engineComp)
	{
	_ecm.CreateComponent(_entity, components::PhysicsEnginePlugin(pluginLib));
	}
	else
	{
	engineComp->SetData(pluginLib,
	[](const std::string &_a, const std::string &_b){return _a == _b;});
	}