Need to support decoding more onion types

[TheBlueMatt]

I received an onion failure after an MPP part timed out which log reported as unknown, i presume we're just out of date compared to BOLTs:

2021-10-17 20:33:13.537 WARN [lightning::ln::onion_utils:484] Onion Error[from ...: (0x17)] Unknown

[dunxen]

I presume that was mpp_timeout? (EDIT: Yup type 23 I see) I think I will be able to tackle this. Will also get me more familiar with onion messages in the spec. Probably are a few other new ones we're missing support for.

[TheBlueMatt]

Yep, the correct type would have been mpp_timeout here. Indeed, I wouldn't be surprised if there were others.