feat!: Have `CustomType`s reference their `Extension` definition #2769

Job	Run time
Check for changes	3s
check	1m 11s
Build benchmarks 🏋️	2m 2s
tests (Rust nightly, coverage)	3m 42s
tests (Rust 1.75)	2m 24s
tests (Rust beta)	2m 5s
tests (Rust stable, all features)	1m 40s
tests (Rust stable, no features)	1m 34s
tests hugr-llvm	0s
Check standard extensions	5s
Required checks 🦀	1s
	14m 47s

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