Add codegen/mock_generator.go

[venediktov]

The format of generators in vanilla-rtb written in Python is not compliant to generators rule e.g. Keys fields followed by value fields and required changes in vanilla-rtb python script.
We need to break dependency of extensions on mock data generation in vanilla-rtb repo

• Add codegen/mock_generator.go

However, If mock_generator.go is very different from the rest of generators under codegen make it a standalone program like bidder_generator.go