feat(py): `Hugr.to_json` and `.load_json` helpers #1706

Job	Run time
Check for changes in Python files	4s
Check serialization schema	20s
check python 3.10	26s
Build HUGR binary	44s
check python 3.12	24s
test python 3.10	25s
test python 3.12	20s
coverage	36s
Required checks 🐍	3s
	3m 22s

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