feature: support talking to etcd via TLS.

[membphis]

The TLS authentication for etcd is usually bidirectional, but cosocket is not supported now, but fortunately, there is currently a PR in progress.

we need to fix this first: openresty/lua-nginx-module#997

related issue: apache/apisix#158

[Jason7602]

so the lua-resty-etcd still do not support connect to etcd via TLS?

[membphis]

we can use etcd proxy, it is an easy way.

[spacewander]

Solved by #86. Server side TLS verification is enough to use by now.