Add skip_timestamps option

[stufro]

Resolves #318

This adds the ability to pass skip_timestamps: true to update and update! methods. To do this I had to change the save method to accept skip_timestamps so to avoid inconsistent behaviour I have also added the option to the create and create! methods.

To achieve this I had to remove the type definition of args on the method definition so that either a Granite::ModelArgs or a NamedTuple can be passed in with skip_timestamps, I originally added another method overload for this case as you can see in the first commit e6f917e. Open to thoughts on this.

I have also updated the crud.md docs to describe this feature and added a missing example for save(validate: false)

[robacarp]

Huh. I have wanted #update with skip_timestamps but never #create. Typically I want that because I want to "soft touch" some record without altering the breadcrumb trail for some reason...which I can't think of why I'd want to do that on a create.

I think the timestamp columns are specified as NOT NULL (or at least they are in my databases). Wouldn't that cause a problem?

[stufro]

Hmm yeah you're right. I hadn't considered DB level constraints.

I've removed it from create and also squashed some of the commits to tidy up the branch.

[crimson-knight]

@stufro I think that you should keep the option to create records while skipping timestamps if the created_at and updated_at values are already present. That's the current gap, creating records in bulk that already have timestamps that want to be preserved but they aren't at the time of creation.

Database constraints will error on their own for the user, so I wouldn't worry about policing that from inside of Granite. I've had instances where we did imports and ignored the existence of the timestamps during creation but allowed them to be updated if/when the record was updated.

[stufro]

@crimson-knight ah yes I suppose that might also help with resolving #481?

I have made the change.