rpc: implement RPC htlc interceptor.

In this commit we add the ability to intercept forwarded htlc packets
straight from the RPC layer. The RPC layer handles a bidrectional stream
that comminucates to the client the intercepted packets and handles its
response by coordinating with the interceptable switch.

lnrpc/routerrpc/router.swagger.json

@@ -844,6 +844,21 @@
         }
       }
     },
+    "routerrpcCircuitKey": {
+      "type": "object",
+      "properties": {
+        "chan_id": {
+          "type": "string",
+          "format": "uint64",
+          "description": "/ The id of the channel that the is part of this circuit."
+        },
+        "htlc_id": {
+          "type": "string",
+          "format": "uint64",
+          "description": "/ The index of the incoming htlc in the incoming channel."
+        }
+      }
+    },
     "routerrpcFailureDetail": {
       "type": "string",
       "enum": [
@@ -885,6 +900,30 @@
     "routerrpcForwardFailEvent": {
       "type": "object"
     },
+    "routerrpcForwardHtlcInterceptRequest": {
+      "type": "object",
+      "properties": {
+        "incoming_circuit_key": {
+          "$ref": "#/definitions/routerrpcCircuitKey",
+          "description": "The key of this forwarded htlc. It defines the incoming channel id and\nthe index in this channel."
+        },
+        "htlc_payment_hash": {
+          "type": "string",
+          "format": "byte",
+          "description": "The htlc payment hash. This value is not guaranteed to be unique per\nrequest."
+        },
+        "amount_msat": {
+          "type": "string",
+          "format": "uint64",
+          "description": "/ The htlc amount."
+        },
+        "expiry": {
+          "type": "integer",
+          "format": "int64",
+          "description": "/ The htlc expiry."
+        }
+      }
+    },
     "routerrpcHtlcEvent": {
       "type": "object",
       "properties": {
@@ -1110,6 +1149,15 @@
     "routerrpcResetMissionControlResponse": {
       "type": "object"
     },
+    "routerrpcResolveHoldForwardAction": {
+      "type": "string",
+      "enum": [
+        "SETTLE",
+        "FAIL",
+        "RESUME"
+      ],
+      "default": "SETTLE"
+    },
     "routerrpcRouteFeeRequest": {
       "type": "object",
       "properties": {