Enable custom disk cache for data publishers

[kean]

Enable disk cache (DataCaching) for data publishers:

let request = ImageRequest(id: "a", data: Just(Test.data))

There are a few options to disable it:

• Create a pipeline with no disk cache (default behavior)
• Implement ImagePipelineDelegate method dataCache(for:pipeline:) or willCache(data:image:for:pipeline:) and disable cache on per-request basis
• Pass .disableDiskCacheWrites option to requests backed by data publishers