buildkit: Handle client cache import options

Cache imports must be explicitly handled by the BuildKit gateway. They are not handled automatically by buildkitd or by dockerfile2llb.

Implementation was basically copy/pasted from upstream's dockerfile frontend.

Note that there are two possible forms of client options in <= v0.10 of the BuildKit client (buildctl and docker buildx). Upstream's master branch, however, normalizing the options into a single form. Once upstream cuts a new minor version and we adopt it, we can remove the old cache-from form.

Bug: T323149