sentinel-dashboard规则持久化相关类能否抽取成一个api项目

## Issue Description

目前sentinel-dashboard只提供了内存的规则存储,自定义只能在sentinel-dashboard项目的源码上修改,这对于后面持续升级sentinel不太方便.希望把规则持久化相关的类抽取成一个api项目

### Describe what happened (or what feature you want)
```xml

>com.alibaba.csp
sentinel-dashboard-api

```

### Describe what you expected to happen

### How to reproduce it (as minimally and precisely as possible)

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### Tell us your environment

### Anything else we need to know?

Start by reviewing the rule-persistence-related classes in sentinel-dashboard and determine the boundary for a separate API project. The issue mentions a sentinel-dashboard-api Maven dependency; done should mean those classes are extractable and usable for customized rule persistence without modifying sentinel-dashboard source.

Written by the indexing model from the issue text.