wallet, rpc: add a release field to get_version

It does not leak much since you can make a fair guess by RPC
version already, and some people want to avoid non release
clients when using third parties' nodes (because they'd never
lie about it)

src/rpc/core_rpc_server.cpp

@@ -2147,6 +2147,7 @@ namespace cryptonote
       return r;
 
     res.version = CORE_RPC_VERSION;
+    res.release = MONERO_VERSION_IS_RELEASE;
     res.status = CORE_RPC_STATUS_OK;
     return true;
   }