Create new UI command to set desired country for ExitService

[czarte]

This command send to Node country_code of desired ExitService location.

masq/src/commands/set_configuration_command.rs .arg(set_configurationify(exit_location_arg())) to set_configuration_subcommand.

[dnwiebe]

I would like to see this command designed so that instead of setting just one desired country, the user can specify a list of them, any one of which would be acceptable. I would also like to see the command designed so that in the future it can be used to specify named country groups (which we don't yet have) with a minimum of effort.

[dnwiebe]

I would also like to see this setting made part of the CONFIG table, and included in the set-configuration command so that it can be set at runtime. Maybe also a new command-line parameter to make it accessible to config.toml files.

[dnwiebe]

Sense from the Sunday meeting is that the Free World Bit and this desired-country-list mechanism should not be allowed to operate at the same time, because they'll interfere with each other. At least one of us has opined that the Free World Bit should be abandoned entirely.

[czarte]

The JSON from UI will be formated as follows:

priority is ordering information. 0 highest and n lowest

{
  "fallback_routing": false,
  "country_codes": [
    {
      "country_codes": ["CZ", ...],
      "priority": 0,
    },
    {
      "country_codes": ["SK", ...],
      "priority": 1,
    }.
    {
      "country_codes": ["PL", ...],
      "priority": 2,
    }
  ]
}

[czarte]

waiting for #545