feat: optional keyv instance injection

[simonecorsi]

This feature allows external libraries that wraps cacheable-request to have control on the keyv cache instance while still defaulting to current behavior.

ref: sindresorhus/got#1523

[coveralls]

Coverage decreased (-1.03%) to 98.974% when pulling 0d4800f on simonecorsi:master into 0656a46 on lukechilds:master.

[simonecorsi]

Coverage decreased (-1.03%) to 98.974% when pulling 0d4800f on simonecorsi:master into 0656a46 on lukechilds:master.

That's unfortunate since the class doesn't return the instance itself to do some unit, gonna check it tonight.

[jaredwray]

@simonecorsi - going to accept this and validate the changes.

[simonecorsi]

Let me know if you want me to open a PR to update the docs 👍

[jaredwray]

@simonecorsi that would be great and if you could also get a fix for unit test.