Expose protocol to get image data from the cache plus WebP cleanup

[andyfinnell]

Summary

Recently we made several headers private in Cocoapods. However, some clients were reaching directly into PINRemoteImage's memory cache (PINCache) and pulling out PINRemoteImageMemoryContainer and doing whatever with it. To support that use case without exposing more than necessary publicly, introduce the PINRemoteImageDataConvertible protocol and have PINRemoteImageMemoryContainer conform to it.

So if previously the code was:

PINRemoteImageMemoryContainer *container = [[PINRemoteImageManager sharedImageManager].cache objectFromMemoryForKey:imageURL.absoluteString];
NSData *cachedImageDataForURL = [container data];

it can be

id<PINRemoteImageDataConvertible> container = [[PINRemoteImageManager sharedImageManager].cache objectFromMemoryForKey:imageURL.absoluteString];
NSData *cachedImageDataForURL = [container data];

Also, previous changes required PIN_WEBP to be defined in the project settings or it would fail to compile. That's been changed to always build. If not specified it will default to 1.

Test plan

Run make all to run tests and build everything