`StateReader` method should not require `&mut self`

[tdelabro]

  fn get_fee_token_balance(
        &mut self,
        contract_address: ContractAddress,
        fee_token_address: ContractAddress,
    ) -> Result<(StarkFelt, StarkFelt), StateError> {
        let low_key = get_fee_token_var_address(contract_address);
        let high_key = next_storage_key(&low_key)?;
        let low = self.get_storage_at(fee_token_address, low_key)?;
        let high = self.get_storage_at(fee_token_address, high_key)?;

        Ok((low, high))
    }

StateReader::get_fee_token_balance should not take &mut self, it's the reader API.

Because this one has a default implem it went under my radar on my PR #1325. It has to be fixed asap

[tdelabro]

I pushed the related changes on our fork in this commit: bidzyyys@4c2580e