support generating separate blob for prefetched files #3321

Job	Run time
nydus-build (amd64)	2m 37s
contrib-build (amd64)	35s
nydus-build (arm64)	2m 23s
contrib-unit-test-coverage	29s
contrib-build (arm64)	40s
nydus-build (ppc64le)	2m 21s
contrib-lint (contrib/nydusify)	55s
nydus-build (riscv64)	2m 26s
contrib-build (ppc64le)	45s
contrib-lint (contrib/nydus-overlayfs)	22s
contrib-build (riscv64)	38s
cargo-deny	43s
nydus-unit-test	5m 9s
nydusd-build-macos (amd64)	3m 5s
nydus-unit-test-coverage	3m 31s
nydusd-build-macos (arm64)	3m 25s
performance-test	0s
nydus-integration-test	0s
takeover-test	0s
upload-coverage-to-codecov	8s
	30m 12s

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