[feature] pkg-config improved UX for custom variables

[SSE4]

there is a pkg_config_custom_content property, but it uses an arbitrary string blob to insert.
it would be nice to have an improved UX to pass a dictionary of variables, just like in CMakeDeps (see #9764, #8360):

self.cpp_info.set_property("pkg_config_variables", {'datadir': '${prefix}/res'}, "pkg_config")
self.cpp_info.components["buy"].set_property("pkg_config_variables",  {'datadir': '${prefix}/res'}, "pkg_config")

a few examples in conan-center:
https://github.com/conan-io/conan-center-index/blob/master/recipes/glib/all/conanfile.py#L300
https://github.com/conan-io/conan-center-index/blob/master/recipes/wayland/all/conanfile.py#L102
https://github.com/conan-io/conan-center-index/blob/master/recipes/xorg-macros/all/conanfile.py#L73

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