perf: Return `Cow<Signature>` where possible #2864

Job	Run time
Check for changes in Python files	11s
check python 3.10	13s
check python 3.13	23s
Check serialization schema	14s
Build HUGR binary	43s
test python 3.10	1m 1s
test python 3.13	56s
Required checks 🐍	1s
	3m 42s

Provide feedback