[cover-image] Pluggable extractor (`extractor: 'ffmpeg' | 'cloudinary' | custom`)

[tsemachh]

Plugin: cover-image · Priority: P2 · Tracked by ROADMAP.md

Problem

Cloud-storage users shouldn't bundle ffmpeg.

Proposed solution

Allow swapping the extractor implementation.

Acceptance criteria

• Built-in ffmpeg extractor.
• Cloudinary adapter scaffolded.
• Custom interface documented.

How to claim

Comment on this issue (e.g. "I'd like to work on this") and a maintainer will assign it to you. Open a PR referencing this issue (Closes #<n>). Discuss design questions here or on the Payload Discord #plugins channel.