feat!: Have `CustomType`s reference their `Extension` definition #2730

Job	Run time
Check for changes	4s
check	1m 6s
Build benchmarks 🏋️	2m 7s
tests (Rust stable, all features)	3m 5s
tests (Rust nightly, coverage)	4m 22s
tests (Rust stable, no features)	1m 23s
tests (Rust 1.75)	2m 35s
tests hugr-llvm	0s
tests (Rust beta)	2m 12s
Check standard extensions	8s
Required checks 🦀	1s
	17m 3s

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