support generating separate blob for prefetched files #3335

Job	Run time
contrib-lint (contrib/nydusify)	1m 0s
contrib-build (amd64)	39s
nydus-build (amd64)	2m 18s
contrib-unit-test-coverage	1m 38s
contrib-lint (contrib/nydus-overlayfs)	30s
contrib-build (arm64)	18s
nydus-build (arm64)	2m 37s
contrib-build (ppc64le)	37s
nydus-build (ppc64le)	2m 10s
contrib-build (riscv64)	46s
cargo-deny	40s
nydus-build (riscv64)	2m 24s
nydusd-build-macos (amd64)	2m 44s
nydusd-build-macos (arm64)	2m 27s
nydus-unit-test	4m 33s
nydus-unit-test-coverage	3m 19s
takeover-test	1m 51s
nydus-integration-test	9m 7s
performance-test (fs-version-5)	2m 6s
performance-test (fs-version-6)	2m 13s
performance-test (zran)	1m 39s
upload-coverage-to-codecov	10s
	45m 46s

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