feat(py): `Hugr.to_json` and `.load_json` helpers #1702

Job	Run time
Check for changes in Rust files	5s
check	0s
tests (Rust nightly, coverage)	0s
Build benchmarks 🏋️	0s
tests (Rust ${{ matrix.rust }})	0s
tests (Rust stable, all features)	0s
tests (Rust stable, no features)	0s
Required checks 🦀	1s
Check standard extensions	0s
	6s

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