Added force option to tools.cmake.cmaketoolchain:extra_variables

[perseoGI]

Changelog: Feature: Added force option to tools.cmake.cmaketoolchain:extra_variables.
Docs: conan-io/docs#3774

Added support for setting FORCE attribute to a cache variable from tools.cmake.cmaketoolchain:extra_variables
Feature originally requested here

Improved error messages to make it more intuitive

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