Set a gas price of 0 for the self-rolled transaction used on gas estimation

[m-Peter]

Since we use a random EOA address to sign a transaction for the eth_estimateGas endpoint, we need to set the GasPrice field to 0 as the EOA address does not have any funds and this will cause a validation error. See: https://github.com/onflow/flow-evm-gateway/blob/main/api/gas_estimation.go#L32. It might be a good idea to also hide the private key used in: https://github.com/onflow/flow-evm-gateway/blob/main/api/gas_estimation.go#L13.