Add etcd username password auth support

[hakur]

Is your feature request related to a problem? Please describe.

I have a etcd cluster with username and password access required, but config-etcd package dose not provide username and password on Options struct .

我有个需要用户名和密码才能访问的etcd集群，但config-etcd包没有在Option结构体上提供用户名和密码

Describe the solution you'd like

add custom etcd client object or provide username password on option struct
在Option字段上添加用户名和密码字段或者直接提供用户自定义实例化好了的的etcd客户端对象

Describe alternatives you've considered

Additional context

[jayantxie]

可以提个 pr 加下