funding+rpc: expand specified chan point shim into a canned assembler

In this commit, we update the `OpenChannel` method to observe the new
`funding_shim` field in the main open channel request. If this is
specified, and is a channel point shim, then we'll create a custom
`chanfunding.Assembler` for the wallet to use in place of the regular
funding workflow.

With this commit, the "initiator" of an external funding flow can now
delegate the remainder of the channel funding workflow to lnd.

server.go

@@ -47,6 +47,7 @@ import (
 	"github.com/lightningnetwork/lnd/lnrpc/routerrpc"
 	"github.com/lightningnetwork/lnd/lnwallet"
 	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
+	"github.com/lightningnetwork/lnd/lnwallet/chanfunding"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/nat"
 	"github.com/lightningnetwork/lnd/netann"
@@ -3108,6 +3109,17 @@ type openChanReq struct {
 
 	// TODO(roasbeef): add ability to specify channel constraints as well
 
+	// chanFunder is an optional channel funder that allows the caller to
+	// control exactly how the channel funding is carried out. If not
+	// specified, then the default chanfunding.WalletAssembler will be
+	// used.
+	chanFunder chanfunding.Assembler
+
+	// pendingChanID is not all zeroes (the default value), then this will
+	// be the pending channel ID used for the funding flow within the wire
+	// protocol.
+	pendingChanID [32]byte
+
 	updates chan *lnrpc.OpenStatusUpdate
 	err     chan error
 }