Support customized parameter parsing logic in parameter flow control

## Issue Description

看了这里关于热点参数限流的描述：
https://github.com/alibaba/Sentinel/wiki/%E7%83%AD%E7%82%B9%E5%8F%82%E6%95%B0%E9%99%90%E6%B5%81

只能指定接口的某个索引位置的参数，不能根据自定义函数解析某个参数的结果来限流，请问这个功能可以支持么？

热点参数限流按对象实体中的某几个具体参数进行限流

Type: *feature request*

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

根据热点参数来限流可以这么做：

```java
ParamFlowRule rule = new ParamFlowRule(resourceName)
.setParamIdx(0)
.setCount(5);
rule.setParamFlowItemList(Collections.singletonList(item));

ParamFlowRuleManager.loadRules(Collections.singletonList(rule));
```

只能指定参数的索引位置，而不能根据自定义函数解析参数的结果来限流。比如如果参数是订单号，后两位是分片位，我想取订单号后2位根据分片来限流。

### Describe what you expected to happen

热点参数限流按对象实体中的某几个具体参数进行限流，或者自定义逻辑判断。

Start with the ParamFlowRule and ParamFlowRuleManager usage shown in the issue, then read the hotspot parameter flow-control documentation linked there. Determine how parameter indexes are currently handled and define how custom extraction or object-field selection should be configured; done means the requested custom parsing behavior is supported and covered by tests.

Written by the indexing model from the issue text.