[3.0] Meaningless/Confusing "key" in dynamic config's yaml

- [x] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.
- [x] I have checked the [FAQ](https://github.com/apache/dubbo/blob/master/FAQ.md) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: 3.0.2-SNAPSHOT
* Operating System version: ANY
* Java version: 8

### Steps to reproduce this issue

服务提供者端，传统服务发现模式，配置中心使用zookeeper或者apollo，注册2个服务
```

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消费者启动后，会在配置中心监节点级别的动态配置和服务级别的动态配置，以zookeeper为例，会监听：

1. 节点级，path=/dubbo/config/dubbo/demo-provider.configurators
2. 服务级，path=/dubbo/config/dubbo/org.apache.dubbo.demo.DemoService:1.2.3:G1.configurators
3. 服务级，path=/dubbo/config/dubbo/org.apache.dubbo.demo.GreetingService:4.5.6:G2.configurators

#### 场景1，节点级配置

向`/dubbo/config/dubbo/demo-provider.configurators`写入DemoService的动态配置
```
configVersion: v2.7
scope: service
key: G1/org.apache.dubbo.demo.DemoService:1.2.3
configs:
- addresses: ['0.0.0.0:20880']
side: provider
parameters:
weight: 222
```
根据提供者reExport日志，发现DemoService和GreetingService都按weight=222进行了重新注册。

#### 场景2，服务级配置

向`/dubbo/config/dubbo/org.apache.dubbo.demo.DemoService:1.2.3:G1.configurators`写入DemoService的动态配置
```
configVersion: v2.7
scope: service
key: AAAAA <--瞎写就行
configs:
- addresses: ['0.0.0.0:20880']
side: provider
parameters:
weight: 333
```
根据提供者reExport日志，发现DemoService按weight=333进行了重新注册。

#### 总结

1. 可以发现yaml中的key没有实际存在的意义，因为key中的服务信息，在DynamicConfiguration实现，以及ConfigChangeListener中都有，在AbstractConfiguratorListener#genConfiguratorsFromRawRule传入即可。此处key属性可取消。
2. 作为scope: service的配置，写入节点级配置路径也能生效，此处最好能控制监听逻辑。节点级只响应scope:application的yaml，服务级只响应scope:service的yaml，或者也可取消scope属性。