As Smart Contract, I should provide the possibility to top up the prize pool

[melpadden]

To make the app look nice from the launch we want to have a possibility to top up the prize pool. The smart contract should provide the top_up_prize_pool entry point, that:

1. Accepts the amount argument
2. Transfers the corresponding amount of CSPR from the caller to the treasury_purse
3. Increases the treasure_purse_balance state variable
4. Increases the prize_pool state variable

This entry point should be called with a help of the WASM proxy.