[PollingResolver] Allow subclasses to utilize the work serializer (#33458)

Child classes of polling resolvers might have use cases where they would want to request a resolution explicitly which honor the min time between requests. As RequestReresolutionLocked needs to be called in from work_serializer which is usually owned by polling resolver completely, allow passing the work serializer back to the child class.

src/core/ext/filters/client_channel/resolver/polling_resolver.h

@@ -69,6 +69,7 @@ class PollingResolver : public Resolver {
   const std::string& name_to_resolve() const { return name_to_resolve_; }
   grpc_pollset_set* interested_parties() const { return interested_parties_; }
   const ChannelArgs& channel_args() const { return channel_args_; }
+  WorkSerializer* work_serializer() { return work_serializer_.get(); }
 
  private:
   void MaybeStartResolvingLocked();