new CMakeConfigDeps

[memsharded]

Changelog: Feature: Make available in conanfiles the new incubating CMakeConfigDeps generator, still under the incubating "conf" feature flag.
Docs: conan-io/docs#3999

[memsharded]

I am proposing this name, because I propose to support only generation of xxx-config.cmake files, not the legacy FindXXX.cmake format for several reasons:

• There are no packages in ConanCenter that use self.cpp_info.set_property("cmake_find_mode", "module")
• There are only 65 packages in ConanCenter that use self.cpp_info.set_property("cmake_find_mode", "both"). There might be consumers of those packages that are happy with config.cmake files.
• Conan private usage is very likely to be 100% config.cmake files, I see very little reasons to want FindXXX.cmake modules besides legacy support for open source third party packages.
• Some of the ConanCenter consumer recipes that rely on a FindXXX.cmake module file might be fixed by some patching to their CMakeLists.txt. If not, the existing CMakeDeps is not going to be retired anytime soon, it can be used in ConanCenter for a long time.
• The logic of the FindXXX.cmake is different enough to be in a dedicated generator. If at some point we want a modern alternative to the CMakeDeps generator to generate FindXXX.cmake modules, we can create a dedicated CMakeFindDeps generator

[jcar87]

!!! LGTM