YITH WooCommerce PDF Invoice 4.16.0

* New: Support for WooCommerce 4.4
* New: Support for WordPress 5.5
* Tweak: make mandatory ssn only for invoices when electronic module is enabled (Italian Customers)
* Update: plugin-fw
* Fix: avoid display array and object meta values in product variations
* Fix: required ssn rule
* Fix: show phone field only if customer has a phone number associated (electronic invoice)
* Dev: replaced deprecated method get_product_from_item
* Dev: changes in the increment invoice number method
* Dev: added the gift card discount as negative

* New: Support for WooCommerce 4.3
* Tweak: validate SSN field only for private customers (Electronic Invoice)
* Update: plugin-fw
* Update: language files
* Fix: fixed displayed warnings in the plugin settings
* Fix: ssn error message showed always
* Dev: added new filter yith_ywpi_ssn_field_placeholder
* Dev: added new filter yith_ywpi_vat_field_placeholder
* Dev: changes the fee tax precision
* Dev: added new filter ywpi_invoice_information_url
* Dev: added new filter ywpi_dropbox_folder
* Dev: added the extension param in some functions

* New: Support for WooCommerce 4.2
* New: French translation
* Update: Language files
* Fix: fixed escaped strings in the plugin settings
* Fix: show the products in the parent order using multi vendor