Add networks to init message

[t-bast]

Implement lightning/bolts#682

This is a good opportunity to setup the infrastructure for a TLV stream in the Init message, which we can leverage for optional, un-speced features.

[codecov-io]

Codecov Report

Merging #1254 into master will increase coverage by 0.5%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #1254     +/-   ##
=========================================
+ Coverage      77%   77.51%   +0.5%     
=========================================
  Files         142      143      +1     
  Lines        9902    10126    +224     
  Branches      389      431     +42     
=========================================
+ Hits         7625     7849    +224     
  Misses       2277     2277

Impacted Files	Coverage Δ
.../fr/acinq/eclair/wire/LightningMessageCodecs.scala	100% <100%> (ø)	⬆️
...-core/src/main/scala/fr/acinq/eclair/io/Peer.scala	76.3% <100%> (+0.56%)	⬆️
...a/fr/acinq/eclair/wire/LightningMessageTypes.scala	86.36% <100%> (+0.64%)	⬆️
...src/main/scala/fr/acinq/eclair/wire/TlvTypes.scala	100% <100%> (ø)	⬆️
.../src/main/scala/fr/acinq/eclair/wire/InitTlv.scala	100% <100%> (ø)
...q/eclair/blockchain/electrum/ElectrumWatcher.scala	51.12% <0%> (-0.6%)	⬇️
...c/main/scala/fr/acinq/eclair/channel/Channel.scala	83.65% <0%> (ø)	⬆️
...rc/main/scala/fr/acinq/eclair/ShortChannelId.scala	100% <0%> (ø)	⬆️
... and 3 more

[pm47]

Nice, love our tlv implementation and the tests are so easy to write. A few minor comments.