support generating separate blob for prefetched files #3344

Job	Run time
contrib-build (amd64)	43s
contrib-build (arm64)	19s
contrib-unit-test-coverage	1m 37s
contrib-build (ppc64le)	44s
contrib-lint (contrib/nydusify)	50s
contrib-build (riscv64)	43s
contrib-lint (contrib/nydus-overlayfs)	29s
nydus-build (amd64)	2m 22s
cargo-deny	39s
nydus-build (arm64)	2m 15s
nydusd-build-macos (amd64)	5m 6s
nydus-build (ppc64le)	2m 13s
nydusd-build-macos (arm64)	3m 46s
nydus-unit-test-coverage	3m 28s
nydus-build (riscv64)	2m 17s
nydus-unit-test	4m 35s
performance-test (fs-version-5)	1m 41s
performance-test (fs-version-6)	1m 46s
nydus-integration-test	9m 25s
takeover-test	3m 59s
performance-test (zran)	1m 49s
upload-coverage-to-codecov	8s
	50m 54s

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