Establishes a relationship between key and item internally in the setting which may be retrieved later.
Should not be used to store passwords or other secrets, which is what add_secret is for.
Before 1.24, item must not be null and not an empty string. Since 1.24, item
can be set to an empty string. It can also be set to null to unset the key. In that case, the behavior
is as if calling remove_data_item.
| this |
the SettingVpn |
| key |
a name that uniquely identifies the given value |
| item |
the value to be referenced by |