Enable gravUSDC in stablemint

To offset bridge risk and diversify the stabletokens in the stablemint module, we propose minting $CMST 1:1 with gravUSDC. With the launch of Harbor, users could deposit different types of stablecoins into stablemint to mint $CMST. The users can deposit their bridged stablecoins into the stablemint and mint $CMST.
This could reduce the stablecoin liquidity fragmentation in the cosmos. In this manner, $CMST can become the central piece of stablecoin liquidity.