Issue #93: Add support for customfield backup/restore #385

Job	Run time
ci / check if job needs to run	5s
ci / prepare test matrix	15s
ci / MOODLE_39_STABLE - mariadb - php 7.2 - mod_cms	3m 6s
ci / MOODLE_311_STABLE - mariadb - php 7.3 - mod_cms	2m 42s
ci / MOODLE_39_STABLE - pgsql - php 7.3 - mod_cms	2m 58s
ci / MOODLE_310_STABLE - pgsql - php 7.2 - mod_cms	3m 8s
ci / MOODLE_39_STABLE - mariadb - php 7.3 - mod_cms	3m 8s
ci / MOODLE_39_STABLE - pgsql - php 7.2 - mod_cms	3m 10s
ci / MOODLE_401_STABLE - pgsql - php 7.4 - mod_cms	3m 3s
ci / MOODLE_400_STABLE - pgsql - php 7.3 - mod_cms	3m 7s
ci / MOODLE_401_STABLE - mariadb - php 7.4 - mod_cms	2m 55s
ci / MOODLE_401_STABLE - pgsql - php 8.0 - mod_cms	3m 53s
ci / MOODLE_401_STABLE - mariadb - php 8.0 - mod_cms	3m 17s
ci / Release to the Moodle plugin directory	0s
	34m 47s

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