some questions while using Sentinel-Zuul-Adapter

## Issue Description

feature request

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

When using Sentinel to protect my zuul gateway, here comes to me some questions.

First of all, I've noticed that `GatewayFlowRule` is just an independent class which is not implement the `Rule` interface while it should be to do so according to our common understood.

Secondlly, when I'm trying to configure the `register2Property` method for each of the 'RuleManagers' such as `GatewayRuleManager`,`DegradeRuleManager`,and `SystemRuleManager`, I found those managers sharing almost the same methods are not extend from any superclasses nor implement any interfaces. So I think common methods should be extracted to make an abstract class or interface then we can get a rather clear view to the family of 'RuleManagers'.

The last thing is, when using Nacos as data persisting tool, I don't know why `GatewayFlowManager` is using `Set` as it's property while other 'Managers' are using `List`, and I have to set an independent method to grab `ReadableDataSource` for GatewayFlowManager only. However, they share exactly the same code but I have to write it twice to meet both `Set` and `List` results.

第一，感觉`GatewayFlowRule`方法应该实现一下`Rule`接口
第二，各种`RuleManager`类是否可以提一个抽象类出来，这样子可能看起来会更清晰一些
第三，在使用nacos持久化流控配置时，为啥唯独`GatewayFlowRule`的property要用Set类型，如此一来当设定网关api的限流，熔断，降级等操作的时候，要为`GatewayFlowRule`单独写一个冗余代码块，这里不知道是否可以优化一下...

### Describe what you expected to happen

1. All the related Rules should be at least an implementation of `Rule`
2. Managers should have a superclass or interface to extract their sharing methods.
3. Make the property type of managers not using different collection type if it is nessessary.

### Tell us your environment
JDK 1.8
latest master branch

### Anything else we need to know?

Start by reading GatewayFlowRule, Rule, GatewayRuleManager, DegradeRuleManager, and SystemRuleManager, including their register2Property methods. Compare the managers' Nacos ReadableDataSource handling and collection types; done means the requested Rule relationship, shared manager design, and collection behavior have a decided, consistent implementation.

Written by the indexing model from the issue text.