feat: add default configs for Gateway and FHEVM

[PacificYield]

This PR includes default config files (in Solidity) that can be inherited to automatically set the addresses to use (1) Zama's FHEVM, (2) Zama's Gateway.

In the future, it may be better to create a standardized repo for any service provider to add their config information.

These configs are meant to be imported such as:

import {MockZamaFHEVMConfig} from "fhevm/config/ZamaFHEVMConfig.sol";

and can be inherited such as:

contract MyEncryptedERC20 is MockZamaFHEVMConfig, EncryptedERC20 {
    /// My code
}