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).

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