Make PluginParams a simple trait

[pm47]

Otherwise we can't extend it in other projects.

[codecov-io]

Codecov Report

Merging #1643 (a413ec6) into master (183bdde) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1643      +/-   ##
==========================================
+ Coverage   85.94%   86.01%   +0.06%     
==========================================
  Files         151      151              
  Lines       11452    11452              
  Branches      512      512              
==========================================
+ Hits         9842     9850       +8     
+ Misses       1610     1602       -8     

Impacted Files	Coverage Δ
.../src/main/scala/fr/acinq/eclair/PluginParams.scala	0.00% <ø> (ø)
...ir-core/src/main/scala/fr/acinq/eclair/Setup.scala	70.52% <100.00%> (ø)
...c/main/scala/fr/acinq/eclair/channel/Channel.scala	85.97% <0.00%> (-0.09%)	⬇️
...nq/eclair/blockchain/electrum/ElectrumClient.scala	74.26% <0.00%> (+0.73%)	⬆️
...q/eclair/blockchain/electrum/ElectrumWatcher.scala	75.39% <0.00%> (+0.79%)	⬆️
...main/scala/fr/acinq/eclair/router/Validation.scala	92.91% <0.00%> (+1.49%)	⬆️
...cala/fr/acinq/eclair/payment/relay/NodeRelay.scala	95.27% <0.00%> (+1.57%)	⬆️