Improve cache and load keys support

[kean]

• Add filteredURL option to ImageRequestOptions

In some cases your image URLs might contains transient query parameters like access tokens which must be ignored when generating cache keys. If that's the case, set this property to a URL without the unwanted fields:

let request = ImageRequest(
    url: URL(string: "http://example.com/image.jpeg?token=1")! ,
    options: .init(filteredURL: "http://example.com/image.jpeg")
)

This option affects both memory and disk caches.

• Refactor the way cache and load keys are created and used