Add a tool for extracting the public key from a signing key

If you have a Ristretto signing key then this gives you a tool for easily finding the public key.

Here's an example:

$ ./result/bin/PaymentServer-get-public-key <ristretto.signing-key 
ogzbCbwIl8qYQkpH1pWoAcJnSpv+UGCHveuEMr471Ew=