Add property to change the CMake variable names generated with CMakeDeps

[juansblanco]

Changelog: Feature: Add cmake_additional_variables_prefixes variable to CMakeDeps generator to allow adding extra names for declared CMake variables.
Docs: conan-io/docs#3721

Created new property for CMakeDeps generator: cmake_legacy_variable_prefix that can be set in package_info() of the conanfile.
It allows changing the prefix used when creating CMake variables instead of the package name that was currently being used.

Closes: #14788

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[memsharded]

I think what we want is the possibility of having multiple variants of the variables, besides the canonical file_name one, the current one. It should be a list that allows different casing/writing of the variable, not just one.

[czoido]

Good job

[valgur]

Just wanted to say thanks for adding this feature. The simple difference in capitalization of variable prefixes has been by far the most common reason for having to apply patches to public CMake projects on CCI. Also the most common reason for having to include custom CMake modules in the recipe. Two birds with one stone. Thanks!