Channel sends typed responses

[t-bast]

Instead of sending strings, channel now sends typed responses.
This is more future-proof when we want to add data to those responses.

TODO:

• E2E tests to verify nothing funky is happening.

[t-bast]

Looks like I have to do some json serialization magic to make it smooth in the CLI, will add that in a separate commit.

[pm47]

Needs merge/rebase

[codecov-io]

Codecov Report

Merging #1321 into master will increase coverage by 0.02%.
The diff coverage is 61.9%.

@@            Coverage Diff             @@
##           master    #1321      +/-   ##
==========================================
+ Coverage   77.59%   77.62%   +0.02%     
==========================================
  Files         144      144              
  Lines       10138    10141       +3     
  Branches      395      406      +11     
==========================================
+ Hits         7867     7872       +5     
+ Misses       2271     2269       -2

Impacted Files	Coverage Δ
...c/main/scala/fr/acinq/eclair/payment/Auditor.scala	95.45% <ø> (ø)	⬆️
...in/scala/fr/acinq/eclair/api/JsonSerializers.scala	95.45% <ø> (ø)	⬆️
...cala/fr/acinq/eclair/db/sqlite/SqliteAuditDb.scala	99.01% <ø> (ø)	⬆️
.../eclair/payment/relay/PostRestartHtlcCleaner.scala	86.13% <0%> (ø)	⬆️
...n/scala/fr/acinq/eclair/channel/ChannelTypes.scala	78.26% <0%> (-11.74%)	⬇️
...q/eclair/payment/receive/MultiPartPaymentFSM.scala	92.5% <0%> (ø)	⬆️
...cinq/eclair/payment/receive/MultiPartHandler.scala	94.56% <0%> (ø)	⬆️
...fr/acinq/eclair/payment/relay/ChannelRelayer.scala	95.06% <100%> (ø)	⬆️
.../scala/fr/acinq/eclair/payment/relay/Relayer.scala	85.36% <100%> (ø)	⬆️
...r/acinq/eclair/payment/send/PaymentLifecycle.scala	92.8% <100%> (ø)	⬆️
... and 5 more