feat!: Have `CustomType`s reference their `Extension` definition #2768

Job	Run time
Check for changes	4s
check	1m 43s
Build benchmarks 🏋️	2m 19s
tests (Rust nightly, coverage)	3m 36s
tests (Rust 1.75)	2m 34s
tests (Rust stable, all features)	2m 22s
tests (Rust beta)	2m 26s
tests hugr-llvm	0s
tests (Rust stable, no features)	1m 41s
Check standard extensions	3s
Required checks 🦀	1s
	16m 49s

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