Relax validation on Duration type in CRDs to include minutes and hours units

[bjee19]

Currently the Duration type in our CRDs have validation to only allow seconds and milliseconds units. This is okay, however for an nginx directive like keepalive_time, the default is 1h. This means if a user were to try and set that nginx directive through a policy CRD, for them to set realistic values, they would need to express a time like 1 hour in seconds which would be inconvenient and confusing.