feat: Do not require `capnp` to be installed to compile `hugr-model` #3405

Job	Run time
Check for changes	3s
check	1m 52s
Build benchmarks 🏋️	2m 58s
Keep hugr-model capnp schema updated	1m 0s
tests (Rust nightly, coverage)	3m 30s
tests (Rust 1.75)	3m 3s
tests (Rust stable, all features)	2m 52s
tests (Rust beta)	2m 43s
tests (Rust stable, no features)	2m 13s
Check standard extensions	4s
Required checks 🦀	1s
	20m 19s

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