Add DNS to TUN, in VPN-Client

[mrpalide]

Did you run make format && make check?

Fixes #993

Changes (Tasks):

• set -dns value on config and read it when vpn-client run.
• modify app put to get -dns value from UI vpn-client setting dialogue
• make change on UI, on skywire vpn ui settings
• make changes on UI, on skywire manager vpn-client setting
  needs to add dns as another parameter in request, like this:
  {"pk":"03c46d47ecf7a0041eff843407312e0c8fad1b140037bbadbec7f38c6547a6dade","passcode":"","dns":"1.1.1.1"}
• add dns set to tun process on linux
• add dns set to tun process on darwin
• add dns set to tun process on windows
• setup skywire dns (maybe SKYDNS)
• put skywire dns value on skywire-utilities skyenv
• put skywire dns value on conf.skywire.dev and conf.skywire.skycoin.com
• get dns value during generate config

How to test this PR:
Generate Config

• just build make build, and generate config skywire-cli config gen -t
• check vpn-client arguments in config file or in VPN UI -> Settings -> Custom DNS Server

Test DNS

• remove dns arguments from vpn-client in config file, should be like:

  {
    "name": "vpn-client",
    "args": [
    	"-srv",
    	"03c46d47ecf7a0041eff843407312e0c8fad1b140037bbadbec7f38c6547a6dade"
    ],
    "auto_start": false,
    "port": 43
  },
  

• start vpn-client
• check dns of TUN (in linux and windows) or Wi-Fi (on darwin)
  • linux: nmcli dev show tun0
  • windows: netsh interface ipv4 show config tun0
  • darwin: networksetup -getdnsservers Wi-Fi or cat /etc/resolv.conf
• check dns by online tools like DNS leak test
• stop vpn-client, add DNS ip to VPN UI -> Settings -> Custom DNS Server value like 1.1.1.1, after that your config should be like this:

   {
    "name": "vpn-client",
    "args": [
    	"-srv",
    	"03c46d47ecf7a0041eff843407312e0c8fad1b140037bbadbec7f38c6547a6dade",
    	"-dns",
    	"1.1.1.1"
    ],
    "auto_start": false,
    "port": 43
  },
  

• start vpn again
• check dns of TUN (in linux and windows) or Wi-Fi (on darwin) again
• check dns by online tools again
• stop vpn
• check dns of Wi-Fi (on darwin) again | In windows and linux TUN driver will delete, so nothing to be check be there

Screencasts:

dns-test-final.mov

[0pcom]

just fix the merge conflicts and we can merge this

[0pcom]

I will come back and fix the flag text. Not sure what the best wording should be.

[0pcom]

hmm, we are setting the DNS per the app here? I suppose that is useful but it makes me think that the DNS is not settable for the visor process itself.

[0pcom]

looks good