Support caching original images on a different cache

[yigitcanyurtsever]

Hey hey 👋,

Here at Zeplin, we needed a way to cache the original images on a different cache (rather than the target cache) when using a custom image processor. For example, we are fetching images with a custom thumbnail image processor, and we want to store these thumbnails on a 'thumbnail' cache, and store the originals on a 'snapshots' cache.

To accomplish this, we wrote a originalCache to KingfisherOptionsInfoItem on our fork. originalCache is basically used for storing and retrieving original images if paired with a non-default image processor. It defaults to targetCache to make it an optional item.

I've also included a typo fix and an optimization to prevent double caching if cacheOriginalImage is selected when used with the default image processor.

Thought this may be a good addition to the main repo. I wasn't able to write or run any tests because of the missing Kingfisher-TestImages folder.

[onevcat]

@yigitcanyurtsever Thanks for it. It looks promising. I would take a look at it later.

BTW, for the testing images, you could take a look at https://github.com/onevcat/Kingfisher#developments-and-tests

[yigitcanyurtsever]

@onevcat Thanks for looking at it! I've updated the PR with some tests.

By the way, I didn't know that it was recommended to open a discussion first – sorry about that.

[onevcat]

@yigitcanyurtsever No problem, bro!

The code looks very nice and I am so glad to merge it. I will prepare an update for it now.
Thank you!