fs: Move file reading helper from composefs-boot #473

Job	Run time
build (bls, rawhide)	6m 45s
build (bls, fedora-compat)	6m 15s
build (bls, arch)	5m 34s
build (bls, rhel9)	7m 14s
build (uki, arch)	9m 9s
build (uki, fedora)	8m 29s
build (bls, fedora)	6m 22s
build (unified, fedora)	8m 48s
build (bls, arch, ext4, none)	4m 43s
build (unified-secureboot, fedora)	7m 45s
build (bls, arch, xfs, none)	5m 34s
	1h 16m 38s

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