Resolve "Add `generated` boolean column for diff files"

What does this MR do and why?

We are adding a generated column to the diff_file table in order to save whether or not a diff is considered a generated file. This is going to allow us to improve the experience when viewing a diff in an MR. With this column we will fill it with data from Gitaly and then we can handle them specially/provide more information to the user about certain files that are auto generated.

Further discussion can be found: #387234 (closed)

Related to #391775 (closed)