Add backup_migrate_backup_memory_limit configuration setting (D8)

Implement backup_migrate_backup_memory_limit setting in the Drupal 8 version like it was very very helpful in D7, see this issue (fixed & commited to 7.x): #2949211: PHP Fatal error: Allowed memory size of 134217728 bytes exhausted →

Rather than increase the PHP memory_limit used by our web site, I would like to conditionally set the memory limit used when Backup Migrate generates database backups. This will give Backup Migrate the memory it needs when database backups are generated, while preserving a low memory limit for all other PHP scripts executed by our web site.