Add Rust code generation for RemoteFunction.thrift (facebookincubator#16458)

Summary:
Pull Request resolved: facebookincubator#16458

This enables Rust thrift types for RemoteFunctionPage and related structs.
Required for Rust sink support in XStream pipelines, since CustomSink.thrift
depends on RemoteFunction.thrift for the RemoteFunctionPage type used in
SinkWriteRequest.

Reviewed By: kunigami

Differential Revision: D93173363

fbshipit-source-id: 9792a4967e45e514feba670ea80ce82d75ece961

Author: cwalv

velox/functions/remote/if/RemoteFunction.thrift

@@ -19,6 +19,7 @@ package "facebook.com/velox/functions"
 namespace cpp2 facebook.velox.functions.remote
 namespace java.swift com.facebook.spark.remotefunctionserver.api
 namespace py3 facebook.velox.functions.remote
+namespace rust velox_functions_remote
 
 cpp_include "folly/io/IOBuf.h"