Get the RP smoothing pool fee recipient dynamically

In this line https://github.com/dappnode/DAppNodePackage-rocketpool-testnet/blob/5d9113ee7935109561f74fad074abb46f6a46aac/build/api/src/index.ts#L64 the fee recipient is hardcoded. Even though the address is supposed not to change, we should be calculating it dynamically