jsonrpc: Add msatoshi argument to sendonion to annotate

While not directly necessary, it still feeds the `listpays` result, and so we
should pass it along if we can, so we don't have to rely solely on the
`amount_sent` field, which includes the fees.

Reported-by: Rusty Russell <@rustyrussell>

plugins/libplugin-pay.c

@@ -1029,6 +1029,7 @@ static struct command_result *payment_createonion_success(struct command *cmd,
 	json_object_end(req->js);
 
 	json_add_sha256(req->js, "payment_hash", p->payment_hash);
+	json_add_amount_msat_only(req->js, "msatoshi", p->amount);
 
 	json_array_start(req->js, "shared_secrets");
 	secrets = p->createonion_response->shared_secrets;