Add an option to specify a custom cache key when using `sd_setImageWithStorageReference`

[erezrokah]

Environment:

• Objective C
• iOS version: 11
• Firebase SDK version: 4.3.0
• FirebaseUI version: 4.1.1
• CocoaPods Version: 1.3.1

Describe the problem:
sd_setImageWithStorageReference uses the storage path as the cache key.
As a result when storing a new image to the same storage path, the new image won't be loaded (the cached image will be loaded).

Steps to reproduce:

1. Store an image in Firebase storage at a specific path.
2. Use sd_setImageWithStorageReference to load it as instructed by documentation.
3. Store a new image in Firebase storage at the same path.
4. Use sd_setImageWithStorageReference to load the image again.
5. Notice that the image from step 2 is loaded.

Observed Results:

• Wrong image is loaded when using sd_setImageWithStorageReference

Expected Results:

• Correct image is loaded when using sd_setImageWithStorageReference

A common solution is to allow a custom cache key.
The glide library has a "Signature" concept:

http://sjudd.github.io/glide/javadocs/360/com/bumptech/glide/signature/MediaStoreSignature.html

[morganchen12]

related to #324

[morganchen12]

Fixed in #654