lnrpc: add a new max_fee field to the CloseChannel RPC call

In this commit, we add a new max_fee field that we'll use to decide when
to bail out of a co-op close dance as the initiator.

lnrpc/lightning.proto

@@ -1910,6 +1910,11 @@ message CloseChannelRequest {
     // A manual fee rate set in sat/vbyte that should be used when crafting the
     // closure transaction.
     uint64 sat_per_vbyte = 6;
+
+    // The maximum fee rate the closer is willing to pay.
+    //
+    // NOTE: This field is only respected if we're the initiator of the channel.
+    uint64 max_fee_per_vbyte = 7;
 }
 
 message CloseStatusUpdate {

lnrpc/lightning.swagger.json

@@ -811,6 +811,14 @@
             "required": false,
             "type": "string",
             "format": "uint64"
+          },
+          {
+            "name": "max_fee_per_vbyte",
+            "description": "The maximum fee rate the closer is willing to pay.\n\nNOTE: This field is only respected if we're the initiator of the channel.",
+            "in": "query",
+            "required": false,
+            "type": "string",
+            "format": "uint64"
           }
         ],
         "tags": [