Add an interface for CacheableMetadata and replace type-hints of RefineableCacheableDependencyInterface with references to it instead

This issue is being reviewed by the kind folks in Slack, #needs-review-queue-initiative. We are working to keep the size of Needs Review queue [2700+ issues] to around 400 (1 month or less), following Review a patch or merge request → as a guide.

Based on the proposed solution this looks like it still needs work.