fs: Move file reading helper from composefs-boot #475

Job	Run time
build (bls, fedora)	6m 48s
build (bls, arch)	5m 13s
build (bls, fedora-compat)	7m 24s
build (bls, rawhide)	7m 11s
build (uki, fedora)	7m 19s
build (unified-secureboot, fedora)	7m 52s
build (unified, fedora)	7m 3s
build (bls, arch, xfs, none)	5m 44s
build (uki, arch)	6m 29s
build (bls, rhel9)	7m 24s
build (bls, arch, ext4, none)	4m 43s
	1h 13m 10s

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