Add channel type feature bit

We already support channel types, but we make it explicit with a feature
bit as required by lightning/bolts#906

eclair-core/src/main/resources/reference.conf

@@ -57,6 +57,7 @@ eclair {
     option_anchors_zero_fee_htlc_tx = disabled
     option_shutdown_anysegwit = optional
     option_onion_messages = disabled
+    option_channel_type = optional
     trampoline_payment = disabled
     keysend = disabled
   }

eclair-core/src/main/scala/fr/acinq/eclair/Features.scala

@@ -208,6 +208,11 @@ object Features {
     val mandatory = 38
   }
 
+  case object ChannelType extends Feature {
+    val rfcName = "option_channel_type"
+    val mandatory = 44
+  }
+
   // TODO: @t-bast: update feature bits once spec-ed (currently reserved here: /~https://github.com/lightningnetwork/lightning-rfc/issues/605)
   // We're not advertising these bits yet in our announcements, clients have to assume support.
   // This is why we haven't added them yet to `areSupported`.
@@ -231,12 +236,13 @@ object Features {
     PaymentSecret,
     BasicMultiPartPayment,
     Wumbo,
-    TrampolinePayment,
     StaticRemoteKey,
     AnchorOutputs,
     AnchorOutputsZeroFeeHtlcTx,
     ShutdownAnySegwit,
     OnionMessages,
+    ChannelType,
+    TrampolinePayment,
     KeySend
   )