lnrpc+rpcserver: encode custom records as custom channel data

With this commit we encode the custom records as a TLV stream into the
custom channel data field of the invoice HTLC.
This allows the custom data parser to parse those records and replace it
with human-readable JSON on the RPC interface.

lnrpc/lightning.proto

@@ -3952,6 +3952,11 @@ message InvoiceHTLC {
 
     // Details relevant to AMP HTLCs, only populated if this is an AMP HTLC.
     AMP amp = 11;
+
+    /*
+    Custom channel data that might be populated in custom channels.
+    */
+    bytes custom_channel_data = 12;
 }
 
 // Details specific to AMP HTLCs.