add L1 client interface to get pending deposits

[jbearer]

Now that we can call async functions on the L1 client during Header::new, we can add a new function:

async fn get_pending_deposits(prev_finalized: Option<u64>, new_finalized: u64) -> Vec<FeeInfo>;

This function should be called from Header:new/Header::from_info whenever the new finalized block is greater than the previous finalized block, and returns a list of fee infos for each Deposit event after the previous finalized block and up to (including) the new finalized block.