Currently, users need to use external tools like age-keygen or rage-keygen to produce encrypted-identity.txt. We should add a zallet command for generating this so it is the only binary required on the system. (Obviously other binaries would be necessary for #252).

The new command should support generating either unencrypted (so access to the file on disk is equivalent to spend authority) or passphrase-encrypted (so zallet rpc walletpassphrase is required to unlock spend authority) identities.