lnrpc: add ability to provide chan point shims for funding, new funding modifiers

In this commit, we start to expose some of the new external funding
functionality over the RPC interface.

First, we add a new `funding_shim` field to the regular `OpenChannel`
method. This can be used by a caller to express that certain parameters
of the funding flow have already been negotiated outside the protocol,
and should be used instead. For example, a shim can be provided to use a
particular key for the commitment key (ideally cold) rather than use one
this is generated by the wallet as normal, or signal that signing will
be carried out in an interactive manner (PSBT based).

Next, we add a brand new method: `FundingStateStep`. FundingStateStep is
an advanced funding related call that allows the caller to either
execute some preparatory steps for a funding workflow, or manually
progress a funding workflow. The primary way a funding flow is
identified is via its pending channel ID. As an example, this method can
be used to specify that we're expecting a funding flow for a particular
pending channel ID, for which we need to use specific parameters.
Alternatively, this can be used to interactively drive PSBT signing for
funding for partially complete funding transactions.

The new transition methods (funding state machine modifiers) in this
commit allow a party to register a funding intent that should be used
for a specified incoming pending channel ID. The "responder" to the
external channel flow should use this to prep lnd to be able to handle
the channel flow properly.

rpcserver.go

@@ -440,6 +440,13 @@ func mainRPCServerPermissions() map[string][]bakery.Op {
 			Entity: "peers",
 			Action: "read",
 		}},
+		"/lnrpc.Lightning/FundingStateStep": {{
+			Entity: "onchain",
+			Action: "write",
+		}, {
+			Entity: "offchain",
+			Action: "write",
+		}},
 	}
 }
 
@@ -5692,3 +5699,17 @@ func (r *rpcServer) BakeMacaroon(ctx context.Context,
 
 	return resp, nil
 }
+
+// FundingStateStep is an advanced funding related call that allows the caller
+// to either execute some preparatory steps for a funding workflow, or manually
+// progress a funding workflow. The primary way a funding flow is identified is
+// via its pending channel ID. As an example, this method can be used to
+// specify that we're expecting a funding flow for a particular pending channel
+// ID, for which we need to use specific parameters.  Alternatively, this can
+// be used to interactively drive PSBT signing for funding for partially
+// complete funding transactions.
+func (r *rpcServer) FundingStateStep(context.Context,
+	*lnrpc.FundingTransitionMsg) (*lnrpc.FundingStateStepResp, error) {
+
+	return nil, fmt.Errorf("not implemented")
+}