Add VLESS seed configurations

[yuhan6665]

@RPRX I think it is ready for review now. I still need to do some test, including compatibility.
I feel this code structure is much better than before. Flexible to implement padding, delay and independent scheduler in the future. Although I don't how to design the user config ;) so currently the config is just on/off and actual values are hard coded and features are basic. But I think the concept should be easy to grasp, e.g:

message Addons {
  string Flow = 1;
  bytes Seed = 2;
  SeedMode Mode = 3;
  string Duration = 4;  // "0-8" means apply to number of packets, "1000b-" means start applying once both side exchange 1kb data, counting two-ways
  PaddingConfig Padding = 5;
  DelayConfig Delay = 6;
  SchedulerConfig Scheduler = 7;
}

enum SeedMode {
  Unknown = 0;
  PaddingOnly = 1;
  PaddingPlusDelay = 2;
  IndependentScheduler = 3;
}

message PaddingConfig {
  uint32 RegularMin = 1;
  uint32 RegularMax = 2;
  uint32 LongMin = 3;
  uint32 LongMax = 4;
}

message DelayConfig {
  bool IsRandom = 1;
  uint32 MinMillis = 2;
  uint32 MaxMillis = 3;
}

message SchedulerConfig {
  uint32 TimeoutMillis = 1; // original traffic will not be sent right away but when scheduler want to send or pending buffer times out
  // Other TBD  
}