Add rake command: `find_users_from_posthog`

What does this MR do and why?

Add a rake script lib/tasks/gitlab/find_users_from_posthog.rake to export user information.

The export process is a 3-step process

1. Based on the Posthog API, export the user id
2. Query the gitlab database to get user information
3. Print user information to standard output in csv format

How to set up and validate locally

$ bundle exec rake gitlab:features:find_users_from_posthog -- --posthog-api-key=******

Performance:

When you Code Review

• In particular, look at whether the code format is standardized
• Is the location of the code files appropriate? 😄

issue: https://jihulab.com/gitlab-cn/gitlab/-/issues/1537