Add etcd data-source extenstion

[louyuting]

Issue Description

Type: feature request
Adapt etcd as dynamic datasource for Sentinel

Describe what feature you want

Need detail design.
PR

Additional context

Add any other context or screenshots about the feature request here.

[zsy619]

从etcd获取sentinel配置信息？

[louyuting]

No, the etcd ACTS as a dynamically configured data center.
You can refer to https://github.com/alibaba/Sentinel/wiki/%E5%8A%A8%E6%80%81%E8%A7%84%E5%88%99%E6%89%A9%E5%B1%95

[wenxuwan]

If there don't have people take it, i can do it.

[louyuting]

Nice~
Contribution are welcomed.

[louyuting]

If there don't have people take it, i can do it.

The issue had been assigned to you.

[louyuting]

If you have good idea, discussion are welcomed.

[wenxuwan]

If you have good idea, discussion are welcomed.

ok, when i finish the design, i will talk with guys in Dingding group.

[louyuting]

Nice

[sczyh30]

FYI: the counterpart in Java (just for reference): https://github.com/alibaba/Sentinel/tree/master/sentinel-extension/sentinel-datasource-etcd

[wenxuwan]

The feature will start after the datasource frame work design finished.