feat: advanced mapping with support for splitting configurations into multiple files.

[Zerorigin]

为advanced mapping添加“支持将配置拆分到多个配置文件”的特性

功能介绍：
1、共用原有的 -c, --config 命令选项，不破坏已有的行为
2、当传参为文件路径时，按照以前的逻辑进行处理
3、当传参为目录路径时，会递归查找指定目录及其子目录(未限制深度，并且会自动跟链接符号)下的json和toml配置文件(支持混合目录)
4、所有配置文件按照advanced mapping的配置尝试进行解析和简单的合并(无去重行为，需使用者自行处理)，解析失败则报错退出程序
5、当传参为目录路径时，且混入LegacyConf配置文件时，解析到此文件会报错退出程序（需使用者自行剔除）
6、当初始化 EndPoint 时，遇到在同一 IP 地址和端口重复监听时，则报错退出（需使用者自行剔除重复项）

[Zerorigin]

#135 #139 这两个 issue 可以随这个 pr 合并后一起关闭

[zephyrchien]

Thanks for your great work and the code looks good to me.

I hesitate to merge this PR because personally I dont think we should spend time on config file stuffs -- It endlessly brings features just for the convenience of some special use case, as well as edge cases which make the codebase hard to maintain.

The current implementation is just a plain & readable view of the internal endpoint::Config, it has basically covered ALL use cases.

[Zerorigin]

Thanks for your great work and the code looks good to me.

I hesitate to merge this PR because personally I dont think we should spend time on config file stuffs -- It endlessly brings features just for the convenience of some special use case, as well as edge cases which make the codebase hard to maintain.

The current implementation is just a plain & readable view of the internal endpoint::Config, it has basically covered ALL use cases.

我希望这个请求被合并。

就像其它 issues 所提到的，其实它们( #108 #129 #135 )旨在解决一个共同问题：
使用 realm 管理太多服务转发端口时，单一配置文件过于冗长而导致的心智压力，毕竟人不是机器。
虽说初次增加配置时可能还好，但后续变更或者删减，不得不在修改配置文件时极其的细心谨慎，这种压力令人很不开心。

另一点正如您所看到的，此段代码的侵入性并不大，合不合并它，都不会导致代码维护的繁琐性和重构的难度明显变大。

它可以作为在 HTTP API 尚未实现的情况下的一种过渡方案，也希望后续在实现 HTTP API 时能考虑到对配置进行分组过滤的情景。

[zephyrchien]

Okay! Let's try to merge this PR then.

[zephyrchien]

@Zerorigin
I made some small modifications. Thank you for putting up this feature!