No option to purge by surrogate keys via Expire module

Okay so ... some background;

https://www.fastly.com/blog/getting-most-out-vary-fastly

TL:DR;

Sometimes when you mess with the cache in various ways, a purge by path isn't possible. But ... we're already adding the path as a surrogate-key anyway, so why not utilise this as a viable alternative?

In implementing this, I also noticed that the soft purge option isn't applied to purgeKeys (https://docs.fastly.com/en/guides/soft-purges) so I've added this here as well.