Method to fetch pending-to-be-sent-but-not-in-HTLCs balance

[TheBlueMatt]

One issue some lightning wallets have is that the balance displayed fluctuates as HTLCs get sent, fail, then get retried. This makes for a pretty strange UX. Sadly, building anything better today with LDK is pretty hard - we now push users to fetch balance using ChainMonitor's Balances, which is great, but they have no idea about any pending outbound payments which have some amount that is waiting to be retried (i.e. is not currently in any HTLCs). We should expose this, maybe via RecentPaymentDetails?