Function fetch_etherscan_source_flattened 

pub async fn fetch_etherscan_source_flattened(
    address: &str,
    etherscan_api_key: &str,
    chain: Chain,
) -> Result<String>

Fetches the source code of a verified contract from Etherscan, flattens it, and returns it.

This provides the same functionality as cast source --flatten but using the library directly, avoiding the need to shell out to the cast binary.