lnwallet: public expose funding keys in channel state machine

This commit modifies the attributes of the LightningChannel struct to
publicly expose the funding keys used within the channel for the local
and remote party. Exposing these keys publicly will allow callers to
use the keys involved to generate authenticated channel advertisements
for the routing layer.

lnwallet/channel.go

@@ -432,6 +432,15 @@ type LightningChannel struct {
 	// counter party has broadcast a prior *revoked* state.
 	ContractBreach chan *BreachRetribution
 
+	// LocalFundingKey is the public key under control by the wallet that
+	// was used for the 2-of-2 funding output which created this channel.
+	LocalFundingKey *btcec.PublicKey
+
+	// RemoteFundingKey is the public key for the remote channel counter
+	// party  which used for the 2-of-2 funding output which created this
+	// channel.
+	RemoteFundingKey *btcec.PublicKey
+
 	started  int32
 	shutdown int32
 
@@ -468,6 +477,8 @@ func NewLightningChannel(signer Signer, bio BlockChainIO,
 		ForceCloseSignal:      make(chan struct{}),
 		UnilateralCloseSignal: make(chan struct{}),
 		ContractBreach:        make(chan *BreachRetribution, 1),
+		LocalFundingKey:       state.OurMultiSigKey,
+		RemoteFundingKey:      state.TheirMultiSigKey,
 	}
 
 	// Initialize both of our chains the current un-revoked commitment for