support generating separate blob for prefetched files #3338

Job	Run time
contrib-unit-test-coverage	1m 37s
contrib-lint (contrib/nydusify)	57s
contrib-build (amd64)	40s
contrib-lint (contrib/nydus-overlayfs)	29s
contrib-build (arm64)	21s
nydus-build (amd64)	2m 27s
cargo-deny	41s
contrib-build (ppc64le)	37s
nydusd-build-macos (amd64)	4m 38s
nydus-build (arm64)	2m 19s
contrib-build (riscv64)	47s
nydusd-build-macos (arm64)	3m 54s
nydus-build (ppc64le)	2m 14s
nydus-unit-test	4m 58s
nydus-build (riscv64)	2m 12s
nydus-unit-test-coverage	3m 37s
takeover-test	1m 59s
performance-test (fs-version-5)	2m 16s
nydus-integration-test	9m 16s
performance-test (fs-version-6)	1m 47s
performance-test (zran)	2m 0s
upload-coverage-to-codecov	10s
	49m 56s

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