lnd+graph: add GraphSource interface and implementation

This commit adds a new GraphSources interface that LND requires for
graph related read-only queries. As of this commit, the interface is
empty but it will be populated over the next couple of commits. We add
an implementation of this interface backed by a pointer to a
graphdb.ChannelGraph.

The infrustructure is put into place so that the GraphSoure provided to
LND can be overridden by a caller of the lnd.Main function. By default,
LND will satisfy the interface itself via the new `graphsource.DBSource`
struct.

config.go

@@ -1802,6 +1802,7 @@ func (c *Config) ImplementationConfig(
 			),
 			WalletConfigBuilder: rpcImpl,
 			ChainControlBuilder: rpcImpl,
+			GraphProvider:       rpcImpl,
 		}
 	}
 
@@ -1813,6 +1814,7 @@ func (c *Config) ImplementationConfig(
 		DatabaseBuilder:     NewDefaultDatabaseBuilder(c, ltndLog),
 		WalletConfigBuilder: defaultImpl,
 		ChainControlBuilder: defaultImpl,
+		GraphProvider:       defaultImpl,
 	}
 }