[API-CHANGE] Bring nouveau Sentinel entry API in Sentinel 2.0 | Sentinel 2.0 全新 API model

[sczyh30]

Issue Description

Type: feature request

Describe what happened (or what feature you want)

There are some drawbacks to legacy Sentinel API model:

• Strategies during invocation (e.g. timeout cut-down, retry, bulkhead isolation) cannot be carried out due to the restriction of the API model (for the lack of "managed" model).
• Lacking capabilities of traffic scheduling (i.e. routing and load balancing).

So we'd like to propose a nouveau Sentinel entry API in Sentinel 2.0.

An example of "managed" model:

try {
  // f (() -> R): your actual logic, wrapped with lamada
  // fallbackSupplier (() -> R): generates the fallback object when fails
  R result = Sentinel.execute(resourceName, f, fallbackSupplier, args);
} catch (BlockException e) {
  // your block handler here
}

An example of "outside-wrapped" model (like legacy entry):

try (Sentinel.entry(resourceName)) {
  // your logic here
} catch (BlockException e) {
  // your block handler here
}

NOTE: The legacy SphU and SphO API will not be deprecated, but we'll recommend using the new API for Sentinel 2.0.

[liaomengge]

Sentinel 2.0相比1.8.x会有很大的变更吗？ 可以将开源的2.0版本直接merge到二开的版本上（基于1.8.x）吗？

[sczyh30]

There will be breaking changes in Sentinel 2.0, but we'll try to keep compatibility with Sentinel 1.8.x.

[liaomengge]

是所有api都发生了很大的变化嘛？ 还是只是部分使用上的改变？ 是类似nacos 1.x到2.x的大变更升级？还是只是增加部分功能以及部分Api名称的变化？

[HanYi001]

希望可以更方便的和配置中心联动，有部分需要修改源码才能和nacos配置中心联动