Support field_config_cardinality

Problem/Motivation

The field_config_cardinality module (https://www.drupal.org/project/field_config_cardinality) allows setting cardinality per field instance (to override field storage settings). It would be great if the Select2 widget could support that, so e.g if an instance of a multiple cardinality field is configured to be single-value only, the widget could be similarly restricted.

Proposed resolution

Attached patch adds support.