[POC] Initial cache for CMake

[franramirez688]

Changelog: Feature: Added a conan_intial_cache[_BUILD_TYPE].cmake file to let CMake build helper run cmake -C conan_intial_cache[_BUILD_TYPE].cmake ... and load all the cache variables (-Dxxxx ones passed CLI).
Docs: https://github.com/conan-io/docs/pull/XXXX

[lasote]

I think the CMake build helper should still read the CMakePresets instead of the new cache file. Some reasons: You still read the self._generator from there, all the information in the Presets is all you need, and maybe in the future we add some configuration to inherit some presets from the user. So, unless we keep upgrading the CMake build helper reading the CMakePresets it won't work.

In addition to that, the purpose of the CMakePresets regarding tools, IDEs, etc is exactly that, parse the json and call cmake accordingly.

Nevertheless, I consider the new file very useful and a simpler way for the consumer in the command line, with CMake version restrictions, to call it easier, but as a "fail safe" feature in case it cannot use Presets.

[lasote]

I still see the new "cache" too coupled to the "CMake" tool and I think they should be isolated even if they share a bit of code for extracting the information from the "CMakePresets".

[lasote]

Looking better!

[czoido]

Finally, we have decided that this should be postponed until we get real feedback of users. The intention of this PR was to cover the case of people using older CMake versions (<3.19) that do not support CMakePresets to let them define the same toolchain cache_variables that are defined in the CMakePresets passing -c initial_cache_vars_file when configuring.
Instead of that, we will output the line with all the -D variables so users can do that manually until we get more feedback about this.