Proposal: support degradation config for Kitex server

[felix021]

Please help add "degradation" as a new type of service governance config to these extensions, instructing Kitex clients to drop the corresponding percentile of requests, which is helpful when the server is confronted with an overwhelming amount of traffic.

For example, the config value may be:

{
    "enabled": true,
    "percentage": 50
}

Meaning the server should drop 50% of the traffic.
The default value should be {"enabled": false} to avoid loss.

Note that Kitex provides a builtin ACL mechanism by client.WithACLRules, which is helpful to implement the degradation policy.

Please prepare a tech plan (lark doc) for this task, including the background, goal, your design details and any problems for discussion before proceeding to the coding phase.

[XiaoYi-byte]

I want to try it.