Add controller and feature flag for code analytics

What does this MR do?

This MR adds a controller, feature flag for code analytics data. I also experiment with input parameter validation using ActiveModel.

Discovery Issue: #12683 (closed)

Implementation issue: #14672 (closed)

How to try it:

1. Enable the feature: Feature.enable(:code_analytics)
2. Visit: /-/analytics/code_analytics
3. You should see an empty page (FE will be implemented here)

Getting Data from the API:

Execute in console:

project = Project.find_by_name!("Gitlab Test")
gemfile = Analytics::CodeAnalytics::RepositoryFile.create!(file_path: 'Gemfile', project: project)
user_model = Analytics::CodeAnalytics::RepositoryFile.create!(file_path: 'app/models/user.rb', project: project)

Analytics::CodeAnalytics::RepositoryFileCommit.create(project: project, commit_count: 2, committed_date: Date.today, analytics_repository_file: gemfile)
Analytics::CodeAnalytics::RepositoryFileCommit.create(project: project, commit_count: 5, committed_date: Date.yesterday, analytics_repository_file: gemfile)
Analytics::CodeAnalytics::RepositoryFileCommit.create(project: project, commit_count: 10, committed_date: Date.yesterday, analytics_repository_file: user_model)

Then visit:

/-/analytics/code_analytics.json?project_id=gitlab-org/gitlab-test&group_id=gitlab-org&file_count=4

Validation error:

/-/analytics/code_analytics.json?project_id=gitlab-org/gitlab-test&group_id=gitlab-org&file_count=1000

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers