Issue #93: Add support for customfield backup/restore #391

Job	Run time
ci / check if job needs to run	4s
ci / prepare test matrix	16s
ci / MOODLE_310_STABLE - pgsql - php 7.2 - mod_cms	3m 12s
ci / MOODLE_311_STABLE - mariadb - php 7.3 - mod_cms	2m 34s
ci / MOODLE_39_STABLE - mariadb - php 7.2 - mod_cms	3m 1s
ci / MOODLE_39_STABLE - pgsql - php 7.2 - mod_cms	3m 3s
ci / MOODLE_39_STABLE - mariadb - php 7.3 - mod_cms	2m 56s
ci / MOODLE_39_STABLE - pgsql - php 7.3 - mod_cms	3m 0s
ci / MOODLE_400_STABLE - pgsql - php 7.3 - mod_cms	2m 59s
ci / MOODLE_401_STABLE - pgsql - php 7.4 - mod_cms	3m 14s
ci / MOODLE_401_STABLE - mariadb - php 7.4 - mod_cms	2m 56s
ci / MOODLE_401_STABLE - pgsql - php 8.0 - mod_cms	3m 16s
ci / MOODLE_401_STABLE - mariadb - php 8.0 - mod_cms	3m 24s
ci / Release to the Moodle plugin directory	0s
	33m 55s

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