Add _createdAt and _updatedAt fields

[glommer]

This PR adds _createdAt and _updatedAt fields. We will establish a rule, that we now check, that ChiselStrike internal entities should start with an underscore.

[glommer]

Draft, because tests don't pass with nested entities.

@BearLemma, I have no idea why and would appreciate your help.

Draft PR is so that we can validate the approach early instead of me bouncing on it for days (which I may not have time to do) so I am tagging some folks that can potentially have feedback:

@penberg @BearLemma @CodingDoug @luisfvieirasilva @honzasp