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

## 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:

```java
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`):

```java
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.