Support the TLS connection when communicating with etcd cluster. We added a configuration item to custom the certificate verification. Whether to setup TLS connection or not depends on the endpoints' scheme, for instance, when endpoints are:
```
etcd:
host:
- "https://127.0.0.1:2379"
- "https://127.0.0.1:3379"
```
APISIX will originate TLS connection automatically, and the Server Name Indication extention will be set by the endpoint host (`127.0.0.1` in above case). Note by default APISIX will verify the certificate, close the verification in configuration explicitly if you want to bypass it.
```
etcd:
tls:
verfiy: false
```
