Decode warning messages

[t-bast]

Add support for logging warning messages as introduced in lightning/bolts#834
Support for sending warning messages instead of current errors will be added in a later PR.
Note that c-lightning already sends these messages, so it will be useful to have them correctly logged.
Even if the spec PR is eventually changed, this will not introduce backwards-compatibility issues so IMO it's worth merging on master now.

[codecov-commenter]

Codecov Report

Merging #1854 (19acba9) into master (85ed433) will decrease coverage by 0.00%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1854      +/-   ##
==========================================
- Coverage   88.94%   88.94%   -0.01%     
==========================================
  Files         150      150              
  Lines       11239    11248       +9     
  Branches      447      449       +2     
==========================================
+ Hits         9997    10004       +7     
- Misses       1242     1244       +2     

Impacted Files	Coverage Δ
...-core/src/main/scala/fr/acinq/eclair/io/Peer.scala	89.58% <0.00%> (-0.95%)	⬇️
...q/eclair/wire/protocol/LightningMessageTypes.scala	84.61% <75.00%> (-1.75%)	⬇️
.../eclair/wire/protocol/LightningMessageCodecs.scala	100.00% <100.00%> (ø)
...main/scala/fr/acinq/eclair/io/PeerConnection.scala	84.68% <0.00%> (-0.43%)	⬇️
...c/main/scala/fr/acinq/eclair/channel/Channel.scala	86.27% <0.00%> (+0.08%)	⬆️
...cinq/eclair/channel/publish/MempoolTxMonitor.scala	81.81% <0.00%> (+1.29%)	⬆️