chore: install deps using FetchContent

[sheny1xuan]

使用cmake的FetchContent 安装依赖，简化编译流程

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[hnwyllmm]

试用了一下，编译会很方便。但是这里有几个问题：

1. 编译libevent的时候，会使用openssl；
2. 每次重新clone下来miniob的代码，在编译的时候，都会去拉取这三个依赖项。这个对miniob的测试平台伤害比较大，可能会导致测试失败。是否有办法在当前环境中已经安装了这些依赖的时候，就不会再查找依赖？