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/script/lua_api/l_object.h

@@ -226,6 +226,12 @@ class ObjectRef : public ModApiBase {
 	// get_breath(self, breath)
 	static int l_get_breath(lua_State *L);
 
+	// set_attribute(self, attribute, value)
+	static int l_set_attribute(lua_State *L);
+
+	// get_attribute(self, attribute)
+	static int l_get_attribute(lua_State *L);
+
 	// set_inventory_formspec(self, formspec)
 	static int l_set_inventory_formspec(lua_State *L);