feat: Add `Const::get_custom_value` #512

Job	Run time
Check for changes in Rust files	3s
tests (Rust stable)	1m 26s
check	54s
benches	1m 41s
tests (Rust 1.75)	1m 29s
tests (Rust beta)	1m 21s
tests (Rust nightly)	1m 35s
Required checks 🦀	1s
coverage	2m 28s
	10m 58s

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