feat: Add `StringVal` to hugr-py #3124

Job	Run time
Check for changes in Python files	4s
Check serialization schema	9s
Build HUGR binary	28s
check python 3.10	12s
check python 3.13	21s
test python 3.10	28s
test python 3.13	28s
Required checks 🐍	1s
	2m 11s

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