[feature] Deprecation of build_requires for 2.X

[lasote]

In Conan 2.0, the name "build_require", when alluding to a tool like CMake (the only meaning it should have as defined in the documentation), will be renamed to tool_require. So to be prepared for the removal, we want to introduce that type mostly as an alias of "build_require".

EDIT: Both the conanfile.txt and the profiles have to allow both of them.