Onion message forwarding

[gijswijs]

With this PR we add basic forwarding functionality for onion messages. It builds on PR #9868.

It adds OnionMessagePayload struct to the lnwire package. It also depends on the not yet merged (PR 68)[https://github.com/lightningnetwork/lightning-onion/pull/68] in the lightning-onion package. For now it uses that package from a forked version.

The msgmux endpoint for onion messages is updated to parse the onion message packet, and forward the onion based on the acquired information.

The SubscribeOnionMessages endpoint is updated to pass along any decrypted information. This endpoint is currently solely meant for itests, although it could have practical use in the future.

[Abdulkbk]

Since https://github.com/lightningnetwork/lnd/pull/9868 has been merged, I think this needs to be updated, wanna take a look.

[lightninglabs-deploy]

@gijswijs, remember to re-request review from reviewers when ready