feat: restructure Part2 database crate #4299

Job	Run time
check	6s
check no_std	4s
check serde	6s
clippy	7s
check no_std kzg-rs	7s
check std	5s
docs	7s
TyposCheck	6s
test stable --no-default-features	17s
fmt	4s
test stable	12s
test stable --all-features	14s
test beta --no-default-features	25s
test beta	24s
test beta --all-features	23s
test nightly --no-default-features	18s
test nightly	18s
test nightly --all-features	18s
	3m 41s

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