should expose get_certkey method

[yxy]

first of all, awesome job. thanks for your kind work 👍 .

I need to check domain certkey exists or not, but I found this method only private.
It would be very convenient if this method can be exposed.

local autossl = require("resty.acme.autossl")
local certkey, err = autossl.get_certkey({domain=ngx.var.server_name, type="rsa"}) 
if certkey and scheme == http then
    ngx.redirect(new_uri, 301)
end

-- I had to hack autoss.lua to make this work

-- make get_certkey visible
AUTOSSL.get_certkey = get_certkey 

[fffonion]

This has been implemented and will be included in next release : )

[fffonion]

@yxy the exposed function is slightly different from internal version, please refer to doc https://github.com/fffonion/lua-resty-acme#autosslget_certkey