Decoupling gateway balance for minting circulating supply

[adlrocha]

Related: #52

Since #52 the gateway is provided with enough balance to mint new tokens whenever a top-down message injects new circulating supply on a subnet. The issue with this is that if there is a vulnerability in the gateway actor, the attacker could drain the whole circulating supply provided for a subnet.

This probably low priority, and we should discuss it further, but to minimize potential attack vectors, we could maybe decouple subnets circulating supply minting from the gateway's basic operation by deploying a new actor that is responsible for minting new funds when certain signals/events are received. This would isolate the token minting logic and the actor managing all the circulating supply for a subnet, making a bit easier finding potential security flaws and attacks.