routing: add option to force import MC pair history

This commit adds the `force` flag to the `XImportMissionControl` RPC
which allows skipping rules around the pair import except for what is
mandatory to make values meaningful. This can be useful for when clients
would like to forcibly override MC state in order to manipulate routing
results.

lnrpc/routerrpc/router.swagger.json

@@ -1702,6 +1702,10 @@
             "$ref": "#/definitions/routerrpcPairHistory"
           },
           "description": "Node pair-level mission control state to be imported."
+        },
+        "force": {
+          "type": "boolean",
+          "description": "Whether to force override MC pair history. Note that even with force\noverride the failure pair is imported before the success pair and both\nstill clamp existing failure/success amounts."
         }
       }
     },