Broken Cmake scripts

### Search before asking

- [x] I searched the [issues](https://github.com/alibaba/async_simple/issues) and found no similar issues.

### What happened + What you expected to happen

Extract from https://www.reddit.com/r/cpp/comments/ta4h29/async_simple_a_c20_coroutine_library/

```
- C++20 support was added in 3.12, this project wrongly requires 3.8
- Pulling in dependencies on the main code path for developer requirements (tests, benchmarks)
- Setting user-settable variables (CMAKE_BUILD_TYPE which is also completely ignored by multi-config generators)
- Hardcoding flags (put them in a toolchain/preset)
- Lack of install interface
```

### Are you willing to submit a PR?

- [ ] Yes I am willing to submit a PR!

The issue does not name a specific CMake file or test. Start by locating the repository's CMake scripts and reviewing how they set the C++ requirement, dependencies, build type, compiler flags, and install rules; done means addressing the listed build-system problems without breaking the supported build configurations.

Written by the indexing model from the issue text.