apollo java客户端的Env.java常量类能支持扩展就好了

[SuperWein]

你的特性请求和某个问题有关吗？请描述

扩展MetaServerProvider的时候发现，getMetaServerAddress参数传的是apollo-core/src/main/java/com/ctrip/framework/apollo/core/enums/Env.java 这个常量类，环境名称都是固定死的，如果我这里有一个SIT环境，读取apollo-env.properties文件的时候会被识别成UNKNOWN，没法像DEV/UAT这样的直接拿到文件中对应环境的meta地址。

public interface MetaServerProvider extends Ordered {
  String getMetaServerAddress(Env targetEnv);
}

public enum Env {
  LOCAL, DEV, FWS, FAT, UAT, LPT, PRO, TOOLS, UNKNOWN;

  public static Env fromString(String env) {
    Env environment = EnvUtils.transformEnv(env);
    Preconditions.checkArgument(environment != UNKNOWN, String.format("Env %s is invalid", env));
    return environment;
  }
}  ```

[nobodyiam]

apollo server 端的 Env 已经改成了常规类而不是 Enum，apollo client 侧应该也能改造，不过工作量比较大

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