feat: Improved representation for metadata in `hugr-model` #3261

Job	Run time
Check for changes	4s
check	1m 58s
tests (Rust nightly, coverage)	3m 46s
tests (Rust 1.75)	3m 1s
Build benchmarks 🏋️	3m 3s
tests (Rust beta)	2m 46s
tests (Rust stable, all features)	2m 34s
tests (Rust stable, no features)	2m 15s
Check standard extensions	4s
Required checks 🦀	1s
	19m 32s

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