Cache key normalize

[Q-RK]

What's the purpose of this PR

修复 config-service 开启缓存后对客户端 app.id 配置大小写敏感的问题

Which issue(s) this PR fixes:

Fixes #3529

Brief changelog

1. 在 ConfigServiceWithCache 中定义 LoadingCache 的装饰者类，负责处理key的大小写问题
2. ConfigServiceWithCache优先取大小写敏感的结果（兼容目前的逻辑）
3. 若区分大小的key获取的值为空，则key转为小写获取对应的值

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[nobodyiam]

I think we need to consider the following situation:

1. The correct appId is hello
2. Client1 access the config service via appId hello
3. Client2 access the config service via appId Hello

BTW, I think they can all get the correct configuration without this pull request (as long as the backend mysql is case insensitive).

After the configuration of hello changes, only client1 can get the updated configuration and client2 will always get the old configuration.

[JaredTan95]

as #3747 merged, please update changs log. :-)

[stale]

This pull request has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in 14 days if no further activity occurs. Please feel free to give a status update now, ping for review, or re-open when it's ready. Thank you for your contributions!

[stale]

This pull request has been automatically closed because it has not had activity in the last 14 days. Please feel free to give a status update now, ping for review, or re-open when it's ready. Thank you for your contributions!