feat: couple convenience functions for nested cache dbs #4491

Job	Run time
check no_std	34s
clippy	1m 22s
check no_std kzg-rs	39s
check	18s
docs	58s
check serde	21s
fmt	6s
check std	18s
test stable --no-default-features	46s
test stable	1m 2s
test stable --all-features	1m 12s
test beta --no-default-features	2m 1s
TyposCheck	7s
test beta	2m 5s
test beta --all-features	2m 9s
test nightly --no-default-features	1m 59s
test nightly	1m 55s
test nightly --all-features	1m 58s
	19m 50s

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