lnrpc: rename proportional fee rate on lnrpc

This field is incorrectly suffixed as "msat", when it is actually
interpreted as the proportional fee rate. This is the value that we
should be using because the sender will calculate proportional fees
accordingly. This is a breaking change to the RPC, but on an
experimental and unreleased API.

lnrpc/lightning.swagger.json

@@ -3523,10 +3523,10 @@
           "format": "uint64",
           "description": "The base fee for the blinded path provided, expressed in msat."
         },
-        "proportional_fee_msat": {
-          "type": "string",
-          "format": "uint64",
-          "description": "The proportional fee for the blinded path provided, expressed in msat."
+        "proportional_fee_rate": {
+          "type": "integer",
+          "format": "int64",
+          "description": "The proportional fee for the blinded path provided, expressed in parts\nper million."
         },
         "total_cltv_delta": {
           "type": "integer",