Add lazy_update_api support

[ofedoren]

This is mostly for Katello/katello#11430, where we try to define params dynamically. Those are also require some data from the database, thus defining them during constant parsing (Rails initialization) is painful (leads to errors since the DB is not ready at that time). Moreover after_initialize doesn't ensure the readiness of the DB, so I've decided to add this lazy block to be evaluated at the time the docs are actually needed.