rpcserver+lnrpc: allow users to update max HTLC channel policies

In this commit, we enable callers of UpdateChannelPolicy to
specify their desired max HTLC forwarding policy for one or
multiple channels.

lnrpc/rpc.proto

@@ -2395,6 +2395,9 @@ message PolicyUpdateRequest {
 
     /// The required timelock delta for HTLCs forwarded over the channel.
     uint32 time_lock_delta = 5 [json_name = "time_lock_delta"];
+
+    /// If set, the maximum HTLC size in milli-satoshis. If unset, the maximum HTLC will be unchanged.
+    uint64 max_htlc_msat = 6 [json_name = "max_htlc_msat"];
 }
 message PolicyUpdateResponse {
 }

lnrpc/rpc.swagger.json

@@ -3150,6 +3150,11 @@
           "type": "integer",
           "format": "int64",
           "description": "/ The required timelock delta for HTLCs forwarded over the channel."
+        },
+        "max_htlc_msat": {
+          "type": "string",
+          "format": "uint64",
+          "description": "/ If set, the maximum HTLC size in milli-satoshis. If unset, the maximum HTLC will be unchanged."
         }
       }
     },