Support for 3-way validation split interface

[hfrick]

closes #697

This PR adds support for the new interface to validation sets in rsample via initial_validation_split() and validation_set().

As laid out in #697 (comment), this

• extends last_fit() to work with objects coming from initial_validation_split()
• adds an argument use_validation_set to last_fit() and fit_best()
• sets defaults for use_validation_set so that we keep using the validation set for objects coming from the old interface but do not use it for objects from the new interface

To make that possible, rsample adds another attribute to validation rset objects coming from the new interface validation_set() in tidymodels/rsample#437

For reference, we will deprecate the old interface: tidymodels/rsample#350

[topepo]

One comment but otherwise great. I'll wait for the name changes from rsample to do anything with this.

[github-actions]

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