Allow use of system fmt

[Tatsh]

Pass -DUSE_SYSTEM_FMT=ON to CMake for this.

[superg]

Want to understand the use case more, does this solve something for you? Current state is that is part of C++20, but it's fully implemented only in MSVC and gcc/llvm don't have it yet so I decided to temporarily add it as a git submodule.

[Tatsh]

On Linux distros any package maintainer will want to link with the system version of any library where possible, for consistency and security reasons.

This is just an option and maintains the prior behaviour.

[superg]

It's not linking anything, when you define FMT_HEADER_ONLY it's using templatized classes. I don't see a need for this when submodule implementation is always newer and is taken directly from the official github fmt repo.