mtg doctor CLI command

[9seconds]

This fixes #351

This PR adds a new CLI command, mtg doctor that validates that everything works as expected. You need to give it a config file, and it will print you a good overview of potential issues with proxy.

$ mtg doctor /etc/mtg.toml 
Deprecated options
  ✅ All good
Time skewness
  ✅ Time drift is -607.048µs, but tolerate-time-skewness is 5s
Validate native network connectivity
  ✅ DC 1
  ✅ DC 2
  ✅ DC 3
  ✅ DC 4
  ✅ DC 5
  ✅ DC 203
Validate network connectivity with proxy socks5://127.0.0.1:1080
  ✅ DC 1
  ✅ DC 2
  ✅ DC 3
  ✅ DC 4
  ✅ DC 5
  ✅ DC 203
Validate fronting domain connectivity
  ✅ xx.xx.xx.xx:yyy is reachable
Validate SNI-DNS match
  ✅ IP address xx.xx.xx.xx matches secret hostname <REDACTED>