lnrpc: add the devrpc development only subserver + devrpc.ImportGraph

This commits adds the devrpc package which implements a subserver that
adds clean separation for RPC calls useful for development and
debugging. This subserver is only compiled in if the dev tag is set.
Furthermore the commit adds the devrpc.ImportGraph call which can
import a graph dump obtained from another node by calling DescribeGraph.
Since the graph dump does not include the auth proofs, the imported
channels will be considered private.

lnrpc/gen_protos.sh

@@ -48,7 +48,7 @@ function generate() {
     --custom_opt="$opts" \
     lightning.proto stateservice.proto walletunlocker.proto
   
-  PACKAGES="autopilotrpc chainrpc invoicesrpc routerrpc signrpc verrpc walletrpc watchtowerrpc wtclientrpc"
+  PACKAGES="autopilotrpc chainrpc invoicesrpc routerrpc signrpc verrpc walletrpc watchtowerrpc wtclientrpc devrpc"
   for package in $PACKAGES; do
     # Special import for the wallet kit.
     manual_import=""
@@ -56,6 +56,11 @@ function generate() {
       manual_import="github.com/lightningnetwork/lnd/lnrpc/signrpc"
     fi
 
+    # Special import for devrpc.
+    if [[ "$package" == "devrpc" ]]; then
+        manual_import="github.com/lightningnetwork/lnd/lnrpc"
+    fi
+
     opts="package_name=$package,manual_import=$manual_import,js_stubs=1,build_tags=// +build js"
     pushd $package
     protoc -I/usr/local/include -I. -I.. \