Add CheckPending method to provider

[mfridman]

Quick PR to add a new provider method to check for pending migrations

func (p *Provider) CheckPending(ctx context.Context) (current, target int64, err error) {}

current is the max db version, and target is the max version from the filesystem.

Implementation is similar to #751. See the bit about not respecting the SessionLocker (if one is configured)

• What is the current version if out-of-order migrations are enabled? Should this method error if that option is enabled or do folks that allow out-of-order migrations already YOLO 🏄‍♂️ and this won't be something that'll matter.