Lays groundwork to make claim_payment* test utils easier to adapt without
changing a million callsites.

We generally allow routing nodes to forward less than the expected HTLC
amount, if the receiver knowingly accepts this and claims the
underpaying HTLC (see `ChannelConfig::accept_underpaying_htlcs`). This
use case is in particular useful for the LSPS2/JIT channel setting where
the intial underpaying HTLC pays for the channel open.

While we previously exposed the withheld amount as
`PaymentClaimable::counterparty_skimmed_fee_msat` on the receiver side,
we did not individually provide it on the forwarding node's side.
Here, we therefore expose this additionally withheld amount via
`PaymentForwarded::skimmed_fee_msat`.