Add new elements to IMPORTABLE list from Supervisor API

[ygoumaz]

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For multiple use-cases, including the one described in https://github.com/cyberbotics/webots_ros2/issues/818, the possibility to add new entries in the IMPORTABLE EXTERNPROTO list at runtime (through the Supervisor API) would allow to automatically define the URL of new PROTOs to import using the existing Supervisor API functions.

An enhancement of the wb_supervisor_field_import_sf_node_from_string and wb_supervisor_field_import_mf_node_from_string with a variable number of arguments to declare all necessary URLs contained in the string could also be considered.