Provide a hook so modules can provide more "derived-from" candidates

Problem/Motivation

SFP fetches (or hotlinks) live files if they miss from dev.

SFP has some logic builtin for the filename relations of core image files.

Now we have some requests to incorporate more file-is-derived-from logic:

- ✨ WebP support - ImageAPI Optimize WebP Fixed
- 🐛 Webp support Fixed

If Drupal keeps succeeding, more will come.

Proposed resolution

My gut feeling is, to keep code maintainable, we have to provide a extension mechanism to encapsulate such logic.

So let's implement a suitable hook.

We can still bikeshed where the implementation should live.

Remaining tasks