Feat add stable tokenc ghs proxy (#536)

### Description

Adds the proxy for GHS which will be used for both cGHS(pilot) and the
actual cGHS in future

### Other changes

no

### Related issues

- mento-protocol/mento-general#576

contracts/tokens/StableTokenGHSProxy.sol

@@ -0,0 +1,7 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+pragma solidity ^0.5.13;
+
+import "celo/contracts/common/Proxy.sol";
+
+/* solhint-disable-next-line no-empty-blocks */
+contract StableTokenGHSProxy is Proxy {}