feat(dns): Support DNS queryStrategy config per NameServer.

[cty123]

There are multiple feature requests already to enable queryStrategy configuration at the granularity of DNS name server: #1953 #1940. The PR adds a field queryStrategy in NameServer struct to allow user to configure DNS query strategy for every NameServer instead of using a single option for all name servers.

The configuration before looks likes this,

"dns": {
  "servers": [
    {
      ... NameServer1
    },
    {
      ... NameServer2
    }
  ],
  "queryStrategy": "UseIP6"
}

After this PR, users are allowed to specify the queryStrategy option for each of the NameServers, for example,

"dns": {
  "servers": [
    {
      ... NameServer1
      +"queryStrategy": "UseIP4"
    },
    {
      ... NameServer2
      +"queryStrategy": "UseIP6"
    }
  ],
  -"queryStrategy": "UseIP6"
}

To be compatible with the old configuration,

1. The old queryStrategy field applied on the DNS config level still works. The NameServer level queryStrategy option can be treated as another filter. This should be pretty straight-forward, for example, in the following case, the queryStrategy would be set to UseIP6,

"dns": {
  "servers": [
    {
      ... NameServer1
      "queryStrategy": "UseIP6"
    },
  ],
  "queryStrategy": "UseIP"
}

but in the following case, the queryStrategy would always result in empty response, due to conflicting DNS option,

"dns": {
  "servers": [
    {
      ... NameServer1
      "queryStrategy": "UseIP6"
    },
  ],
  "queryStrategy": "UseIP4"
}

Therefore, users are encouraged to use NameServer level queryStrategy field.

2. If queryStrategy is not set at NameServer level, the NameService will not override the queryStrategy setting. The behavior of the DNS resolver should remain the same as previous.

[yuhan6665]

麻烦看一下 TestLocalNameServerWithIPv6Override 在 windows 这个测试

[yuhan6665]

Do you think it make sense to extract this method to a common place?

[cty123]

Created a more general function under nameserver.go

[cty123]

麻烦看一下 TestLocalNameServerWithIPv6Override 在 windows 这个测试

可能跟windows下go的DNS lookup实现有关系，macos和linux都能过这个测试。稍微看了下源码，

       if r.preferGoOverWindows() {
		return r.goLookupIP(ctx, network, name)
	}
	// TODO(bradfitz,brainman): use ctx more. See TODO below.

	var family int32 = syscall.AF_UNSPEC
	switch ipVersion(network) {
	case '4':
		family = syscall.AF_INET
	case '6':
		family = syscall.AF_INET6
	}
...

也有可能跟运行环境有关系，但是目前手上暂时没有windows的机器，所以没法测试。所以暂时移除了在local nameserver使用queryStrategy的功能。

[yuhan6665]

感谢佬！