support generating separate blob for prefetched files #3339

Job	Run time
contrib-unit-test-coverage	1m 34s
contrib-build (amd64)	47s
contrib-lint (contrib/nydusify)	1m 2s
contrib-build (arm64)	18s
contrib-lint (contrib/nydus-overlayfs)	27s
contrib-build (ppc64le)	45s
nydus-build (amd64)	2m 34s
contrib-build (riscv64)	46s
nydus-build (arm64)	2m 27s
nydus-build (ppc64le)	2m 10s
cargo-deny	40s
nydusd-build-macos (amd64)	2m 50s
nydus-build (riscv64)	2m 10s
nydusd-build-macos (arm64)	3m 38s
nydus-unit-test	4m 39s
nydus-unit-test-coverage	3m 27s
nydus-integration-test	9m 11s
takeover-test	2m 12s
performance-test (fs-version-5)	1m 48s
performance-test (fs-version-6)	1m 40s
performance-test (zran)	1m 42s
upload-coverage-to-codecov	14s
	47m 1s

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