Add C++ module support

[mikomikotaishi]

This PR adds support for C++20 modules, using option CPR_BUILD_MODULES, which builds a module cpr.

[mikomikotaishi]

@COM8 Do you know anything about the failing tests? I haven't touched anything relating to those why is why I'm confused about it, nothing I touched should have any effect on the tests

[COM8]

@mikomikotaishi thanks for contributing! Yes this is a welcome suggestion.

Looks like clang-tidy, cmake and cppcheck updated. Those are not your issues there. I will create a MR in the next couple of days to deal with them.

I have a couple of requests:

1. Please change the cmake_minimum_version to cmake_minimum_required(VERSION 3.15...4.1) so we can enable more default policies if available.
2. Should we add some integration test to ensure it works?
3. Can we already use import std;?
4. You are implementing the module support "the clang way" by taking care of using. I personally prefer the MSVC way of doing it with adding a macro in front of everything (https://youtu.be/7WK42YSfE9s?si=ulBcp6Y2BA9PzN9r&t=2247). For me this has the advantage that we do not have to maintain this separate file with all includes and instead we keep changes close to our code.
5. I personally prefer the file extension .cxx since it is only 3 characters and to me if is the logical successor of .hpp and .cpp.
6. Do we need set(CMAKE_CXX_SCAN_FOR_MODULES ON)?
7. We need a CI run for this to test the build.
8. Are there other modules we can consume e.g. gtest and curl?
9. Why do we need the explicit inline (https://clang.llvm.org/extra/clang-tidy/checks/readability/redundant-inline-specifier.html)?

[mikomikotaishi]

To answer your questions:

• We could certainly add CI tests, I have seen them before, but I don't know much about it before.
• import std; doesn't work in CMake in my experience. It's hidden behind an experimental flag which requires a UUID or some string that has to be found online, and changes each update, if I recall correctly. I think it's possible if you manually compile it and link it, which is plausible for small projects (a single file that imports std, for example)
• It seems like a very herculean task to migrate the whole project into the MSVC-style, but I'm not against it. Could we do it in a subsequent PR, perhaps?
• I haven't used set(CMAKE_CXX_SCAN_FOR_MODULES ON) before, so I don't think we need it to my knowledge, but if you know more please tell me.
• I don't know if gtest has a module, and if I am understanding your question libcurl isn't modularisable due to being C. Please do clarify if I am not understanding correctly.
• I assume you are asking about <cpr/status_codes.h>, these constants have to be inline because they need external linkage to be exported from a module.

Everything else, I can make the changes now.

[mikomikotaishi]

@COM8 Hi, I'm sorry for the long delay. I've made a CI for building the module, but I don't imagine there is any need to make one to "test" the features of the modules, as it is just a re-export of the library.

[COM8]

@mikomikotaishi thanks for making the requested changes. If you take care of rebasing, I can take care of migrating it to the MSVC way of doing it.

[mikomikotaishi]

OK, I've rebased.