Custom fields for activity modules

This plugin adds Custom Fields to all activity modules. Custom fields API was introduced in Moodle 3.7 and adds a standard way to support this feature in many contexts. With this plugin, the site admin can create fields at site level that will be displayed on the forms of all activity modules. Currently, there is only support to manage the fields and save their data in each activity module for every course (it uses the course module id). In future we can display theses fields wherever we want.