feat: Add `StringVal` to hugr-py (#1818) #3128

Job	Run time
Check for changes in Python files	4s
Build HUGR binary	38s
check python 3.10	11s
check python 3.13	23s
Check serialization schema	7s
test python 3.10	22s
test python 3.13	29s
Required checks 🐍	1s
	2m 15s

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