support generating separate blob for prefetched files #3299

Job	Run time
nydusd-build-macos (amd64)	5m 51s
contrib-build (amd64)	33s
nydusd-build-macos (arm64)	5m 33s
contrib-build (arm64)	38s
contrib-unit-test-coverage	1m 18s
contrib-build (ppc64le)	47s
contrib-lint (contrib/nydusify)	1m 3s
contrib-build (riscv64)	48s
contrib-lint (contrib/nydus-overlayfs)	30s
nydus-build (amd64)	3m 42s
cargo-deny	43s
nydus-build (arm64)	3m 42s
nydus-unit-test	5m 23s
nydus-build (ppc64le)	3m 41s
nydus-unit-test-coverage	4m 2s
nydus-build (riscv64)	3m 25s
performance-test	0s
takeover-test	0s
nydus-integration-test	0s
upload-coverage-to-codecov	39s
	42m 18s

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