`prune`: filtering out `ADD --checksum=... https://...` entries

[fenollp]

Description

I'd like to be able to run docker buildx prune but keep in cache the pinned ADD downloads. Could we extend on the --filter=.. syntax?

Side curiosity: why are these cache entries marked Mutable: true? From my understanding of du's docs that'd mean that its size can grow.

Is there a missed optimization opportunity of in the ADD op here, setting mutable=false? cc https://github.com/moby/buildkit/issues/975#issuecomment-490586307