Implement player attribute backend (#4155)

* This backend permit mods to store extra players attributes to a common interface.
* Add the obj:set_attribute(attr, value) Lua call
* Add the obj:get_attribute(attr) Lua call

Examples:
* player:set_attribute("home:home", "10,25,-78")
* player:get_attribute("default:mana")

Attributes are saved as a json in the player file in extended_attributes
key

They are saved only if a modification on the attributes occurs and loaded
when emergePlayer is called (they are attached to PlayerSAO).

src/content_sao.cpp

@@ -791,6 +791,7 @@ PlayerSAO::PlayerSAO(ServerEnvironment *env_, u16 peer_id_, bool is_singleplayer
 	m_pitch(0),
 	m_fov(0),
 	m_wanted_range(0),
+	m_extended_attributes_modified(false),
 	// public
 	m_physics_override_speed(1),
 	m_physics_override_jump(1),