support generating separate blob for prefetched files #3337

Job	Run time
contrib-unit-test-coverage	1m 44s
contrib-lint (contrib/nydusify)	58s
contrib-lint (contrib/nydus-overlayfs)	26s
contrib-build (amd64)	45s
nydus-build (amd64)	2m 23s
contrib-build (arm64)	14s
nydus-build (arm64)	2m 15s
contrib-build (ppc64le)	41s
nydus-build (ppc64le)	2m 13s
cargo-deny	40s
contrib-build (riscv64)	44s
nydusd-build-macos (amd64)	4m 50s
nydus-build (riscv64)	2m 15s
nydusd-build-macos (arm64)	3m 17s
nydus-unit-test-coverage	3m 22s
nydus-unit-test	4m 43s
nydus-integration-test	9m 17s
takeover-test	2m 4s
performance-test (fs-version-5)	1m 43s
performance-test (fs-version-6)	1m 45s
performance-test (zran)	1m 43s
upload-coverage-to-codecov	10s
	48m 12s

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