look for a way to define custom rules for given param

## Issue Description

Type: *feature request*

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

i using sentinel zuul adapter for my zuul ratelimit control

i hope this is a way to define custom rules for some given param

for example：

aiming at ip control rule ,the basic rule is 50 qps allowed per seconds, but in a group the extranet ip may be the same, or pressue test ,request send from the same ip address; this may cause BlockException

```
Set rules = new HashSet<>();
GatewayParamFlowItem paramFlowItem = new GatewayParamFlowItem();
paramFlowItem.setParseStrategy(SentinelGatewayConstants.PARAM_PARSE_STRATEGY_HEADER);
paramFlowItem.setFieldName("uip");
rules.add(new GatewayFlowRule("myrouteid")
.setCount(50)
.setIntervalSec(1)
.setControlBehavior(RuleConstant.CONTROL_BEHAVIOR_DEFAULT)
.setParamItem(paramFlowItem)
);
```

can GatewayFlowRule provide 'customParamItems' or someting else to set target rule threshold,

this 'customParamItems' List ,GatewayParamFlowItem may hold a 'count' filed as flow control threshold, if request param match some of the 'customParamItems' item ,using its count to achieve custom rules purpose ,if not ,using the default

### Describe what you expected to happen

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

1.
2.
3.

### Tell us your environment

```
sentinel-zuul-adapter 1.6.3
sentinel 1.6.3
```

### Anything else we need to know?

Start by reviewing GatewayFlowRule and GatewayParamFlowItem in the sentinel-zuul-adapter 1.6.3 integration, focusing on how the header field uip is parsed and how the default threshold is applied. Define how custom parameter matches and per-item count thresholds should interact with the existing default, then verify the behavior in the Sentinel gateway flow-control tests.

Written by the indexing model from the issue text.