feat: extract statetest models/structs to standalone crate #4360

Job	Run time
check no_std	22s
clippy	1m 17s
check no_std kzg-rs	38s
check	16s
check serde	20s
docs	1m 7s
check std	21s
fmt	5s
test stable --no-default-features	54s
test stable	1m 4s
TyposCheck	7s
test stable --all-features	1m 6s
test beta --no-default-features	57s
test beta	49s
test beta --all-features	1m 14s
test nightly --no-default-features	1m 6s
test nightly	46s
test nightly --all-features	1m 8s
	13m 37s

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