Added DoH support GET/POST

[pr4u4t]

This adds DoH support with POST and GET requests, this requests could be cached.

local r, err = resolver:new{
nameservers = { "https://cloudflare-dns.com/dns-query" },
retrans = 5, -- 5 retransmissions on receive timeout
timeout = 2000, -- 2 sec
no_random = true, -- always start with first nameserver,
doh = 'POST'
}

[xiaocang]

And the DoH client should provide an option to setting up bootstrap DNS

[pr4u4t]

And the DoH client should provide an option to setting up bootstrap DNS

Should I add bootstrap DNS?

[zhuizhuhaomeng]

And the DoH client should provide an option to setting up bootstrap DNS

Should I add bootstrap DNS?

you need to add test cases for the new feature. And the bootstrap DNS option is still needed.