support generating separate blob for prefetched files #3322

Job	Run time
contrib-unit-test-coverage	33s
contrib-lint (contrib/nydusify)	50s
nydus-build (amd64)	2m 33s
contrib-lint (contrib/nydus-overlayfs)	27s
contrib-build (amd64)	35s
nydus-build (arm64)	2m 29s
contrib-build (arm64)	45s
cargo-deny	39s
contrib-build (ppc64le)	38s
nydus-build (ppc64le)	2m 26s
nydusd-build-macos (amd64)	4m 0s
contrib-build (riscv64)	39s
nydus-build (riscv64)	2m 12s
nydusd-build-macos (arm64)	4m 31s
nydus-unit-test	4m 36s
nydus-unit-test-coverage	3m 32s
takeover-test	2m 8s
nydus-integration-test	9m 11s
performance-test (fs-version-5)	1m 58s
performance-test (fs-version-6)	1m 41s
performance-test (zran)	1m 48s
upload-coverage-to-codecov	9s
	48m 20s

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