Provide mechanism to add custom project variables into generated conanbuildinfo.* file

I did not find how to pass a custom project variable into the generated conanbuildinfo.* file.

I have a library that builds DLLs with a suffix (based on version and some other info) like ‘file_1_22xyz.dll’. It will be handy to define in a conanfile.py some variables that will be passed into a ‘conanbuildinfo’. For instance:

# conanfile.py
class MyLibConan(ConanFile):
    def package_info(self):
        self.project_info.vars = [ 'MYLIB_SUFFIX=1_22xyz' ]

# conanbuildinfo.pri
MYLIB_SUFFIX = 1_22xyz

[memsharded]

#1477 implements user_info_vars that provides this functionality. Will be in 0.25

[lasote]

#1477 for now, txt and cmake generators will be available.