Get supply at

Returns the supply at a given time (as Unix time) in Lunas (1 NIM = 100,000 Lunas). It is calculated using the following formula: ```text supply(t) = total_supply - (total_supply - genesis_supply) * supply_decay^t ``` Where t is the time in milliseconds since the PoS genesis block and `genesis_supply` is the supply at the genesis of the Nimiq 2.0 chain.