Draft: Add fallback solution for std::format when building on macOS 13 Ventura

This MR introduces a fallback solution for std::format when building the most recent GeoModel codebase on macOS 13 Ventura or other platforms that support only an incomplete set of C++20 features.

The fallback is mostly transparent to the end user: CMake requests C++20 as the default version, then checks if std::format is supported; if not, it search for a local installation of the fmt library; if not found, the latest version of the fmt library is fetched during the build, just for the compilation.

A Note: fmt is the library that defined the format feature, whose code was merged into C++20 as std::format. Therefore, the fmt library is a de facto 1:1 replacement for std::format when it is missing on a specific platform.

Tested on macOS 13.7