support generating separate blob for prefetched files #3309

Job	Run time
nydusd-build-macos (amd64)	3m 51s
contrib-lint (contrib/nydusify)	53s
contrib-lint (contrib/nydus-overlayfs)	23s
nydusd-build-macos (arm64)	2m 42s
contrib-unit-test-coverage	1m 4s
contrib-build (amd64)	34s
contrib-build (arm64)	49s
contrib-build (ppc64le)	48s
nydus-unit-test	4m 49s
contrib-build (riscv64)	49s
cargo-deny	41s
nydus-build (amd64)	2m 53s
nydus-unit-test-coverage	3m 48s
nydus-build (arm64)	2m 46s
nydus-build (ppc64le)	2m 35s
nydus-build (riscv64)	2m 28s
takeover-test	1m 26s
performance-test (fs-version-5)	1m 17s
nydus-integration-test	7m 34s
performance-test (fs-version-6)	1m 20s
performance-test (zran)	1m 48s
upload-coverage-to-codecov	14s
	45m 32s

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