Add a serializer for DoSync

[pm47]

This command is sent by the Peer to its PeerConnection, therefore it
needs to be a RemoteType and have its own codec.

[codecov-io]

Codecov Report

Merging #1708 (0bc440c) into master (5d662fc) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1708      +/-   ##
==========================================
+ Coverage   86.27%   86.30%   +0.02%     
==========================================
  Files         151      151              
  Lines       11699    11701       +2     
  Branches      506      501       -5     
==========================================
+ Hits        10093    10098       +5     
+ Misses       1606     1603       -3     

Impacted Files	Coverage Δ
...main/scala/fr/acinq/eclair/io/PeerConnection.scala	83.40% <ø> (+0.42%)	⬆️
...cinq/eclair/remote/EclairInternalsSerializer.scala	97.50% <100.00%> (+0.06%)	⬆️
...cala/fr/acinq/eclair/payment/relay/NodeRelay.scala	93.70% <0.00%> (-1.58%)	⬇️
...main/scala/fr/acinq/eclair/router/Validation.scala	92.30% <0.00%> (+1.53%)	⬆️