lnrpc+rpcserver: Add blinded payment paths to PayReq

This commit adds a blinded_paths field to the PayReq proto message. A
new helper called `CreateRPCBlindedPayments` is then added to convert
the zpay32 type to the existing `lnrpc.BlindedPaymentPath` type and add
this to the `PayReq` in the `DecodePayReq` rpc method.

lnrpc/lightning.proto

@@ -4289,6 +4289,7 @@ message PayReq {
     bytes payment_addr = 11;
     int64 num_msat = 12;
     map<uint32, Feature> features = 13;
+    repeated BlindedPaymentPath blinded_paths = 14;
 }
 
 enum FeatureBit {