FindPkgConfig: Fix parsing of quoted lists with pkgconf <1.5.1
Unquote pkg-config output if they are printed within quotes. pkgconf <1.5.1 and classic pkg-config <0.29.1 prints quoted variables without unquoting them, this breaks returning variables with multiple values as a list behavior of CMake.
Add a new test case for pkg_get_variable with multiple values to test list behavior and backslash escaped spaces within variable values.
Fixes: #25904 (closed)