As part of implementing async payments, we need a way to store onion message forwards on behalf of often-offline next-hop peers. This allows payers to send a held_htlc_available onion message to the mobile recipient, where the recipient's counterparty holds onto this onion message until the recipient comes back online.

Rather than storing onion messages internally in LDK, we generate events on OM interception and later signal to the user to re-inject these onion messages for forwarding.

This implements basic offline peer OM interception support. As a follow-up, we should support indicating to users when an intercepted OM has successfully been forwarded and is safe to delete, see #2950.

Partially addresses #2950, partially addresses #2298.