API: Reflection Service Support

[Vigilans]

This PR implements a new service ReflectionService, which is a simple wrapper of grpc/reflection package:

  "api": {
    "tag": "api",
    "services": ["ReflectionService"]
  }

This service enables the clients to retrieve gRPC service's API list and signatures without prior access to its proto files. In this way, tools like grpcurl could easily interact with the API service:

# h2c
$ grpcurl -plaintext localhost:10086 list
grpc.reflection.v1alpha.ServerReflection
v2ray.core.app.proxyman.command.HandlerService
v2ray.core.app.stats.command.StatsService

# h2 with tls
$ grpcurl -insecure -cert cert.crt -key cert.key localhost:10086 v2ray.core.app.stats.command.StatsService/GetSysStats
{
  "NumGoroutine": 24,
  "NumGC": 25,
  "Alloc": "24095640",
  "TotalAlloc": "511473656",
  "Sys": "146657288",
  "Mallocs": "6909875",
  "Frees": "6464105",
  "LiveObjects": "445770",
  "PauseTotalNs": "1834456",
  "Uptime": 82
}

Without the reflection service, a proto file has to be provided to retreive service and API's signature information:

$ grpcurl -import-path core/app/stats/command -proto command.proto -plaintext localhost:10086 describe v2ray.core.app.stats.command.StatsService
v2ray.core.app.stats.command.StatsService is a service:
service StatsService {
  rpc GetStats ( .v2ray.core.app.stats.command.GetStatsRequest ) returns ( .v2ray.core.app.stats.command.GetStatsResponse );
  rpc GetSysStats ( .v2ray.core.app.stats.command.SysStatsRequest ) returns ( .v2ray.core.app.stats.command.SysStatsResponse );
  rpc QueryStats ( .v2ray.core.app.stats.command.QueryStatsRequest ) returns ( .v2ray.core.app.stats.command.QueryStatsResponse );
}

This service has to be specified in API config manually to take effect, since it exposes the detailed API information to clients without any prior knowledge.