perf: Return `Cow<Signature>` where possible (#1743) #2866

Job	Run time
Check for changes in Python files	4s
Build HUGR binary	45s
check python 3.10	13s
check python 3.13	23s
Check serialization schema	14s
test python 3.10	22s
test python 3.13	29s
Required checks 🐍	1s
	2m 31s

Provide feedback