A pluggable image loader, with quality and unoptimized

[romain-pm]

Part of #734. Resize routing is decided inside buildImageUrl and a module cannot override it.

buildImageUrl picks the channel (provider, thumbnail, query, original) from the node's provider. A project on a CDN, a custom DAM, or a Media Optimization setup that speaks a different URL dialect has no way in.

• loader?: (props: { src: string; width: number; quality?: number }) => string, per call and configurable module-wide. This is next/image's ImageLoaderProps exactly, and it is Astro's pluggable image service in a smaller package.
• quality, passed to the loader. No knob exists today.
• unoptimized, to opt a single image out of candidate generation — Next's escape hatch, Astro's passthroughImageService at call-site granularity.
• Keep the reported channel (inspectImageChannel) meaningful when a custom loader is in play.