Add json type hints on channel data

[pm47]

This is particularly helpful when in OFFLINE state.

[codecov-commenter]

Codecov Report

Merging #1824 (d6a6f26) into master (98cae45) will decrease coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1824      +/-   ##
==========================================
- Coverage   89.39%   89.31%   -0.09%     
==========================================
  Files         145      145              
  Lines       11034    11034              
  Branches      456      456              
==========================================
- Hits         9864     9855       -9     
- Misses       1170     1179       +9     

Impacted Files	Coverage Δ
...clair/blockchain/bitcoind/rpc/BatchingClient.scala	86.36% <0.00%> (-13.64%)	⬇️
...lockchain/bitcoind/rpc/ExtendedBitcoinClient.scala	92.85% <0.00%> (-3.58%)	⬇️
...in/scala/fr/acinq/eclair/channel/TxPublisher.scala	95.23% <0.00%> (-1.79%)	⬇️
...main/scala/fr/acinq/eclair/router/Validation.scala	90.76% <0.00%> (-1.54%)	⬇️
...c/main/scala/fr/acinq/eclair/channel/Channel.scala	86.41% <0.00%> (ø)
...cala/fr/acinq/eclair/payment/relay/NodeRelay.scala	97.65% <0.00%> (+1.56%)	⬆️