Add dependency manager

Currently only SIMDJSON is used as a submodule. But in the future there may be other dependencies (for example, a third-party HTTP server or exclude getopt from the codebase).
Submodules are very inconvenient, not very effective, and tie the project to a version control system, which should not be the case.

Probably better solution will be use dependency manager. [CPM](https://github.com/cpm-cmake/CPM.cmake) is lightweight and written in pure CMake.