mina-signer: use string instead of boolean for compilation option

[nicc]

We need to make the mina-signer change that goes with this PR: MinaProtocol/mina#13232

[bkase]

Context: If we don't expose this in mina-signer, it's not really real. Why is this feature important? It prevents replay attacks from an arbitrary string-labeled network (instead of just two, which isn't enough in a world with projects like zeko)