Add iOS-fastlane template for .gitlab-ci.yml

What does this MR do?

Adds a template that aligns to the blog post about publishing iOS using fastlane, as a way to discover that ability.

This should not be live until the blog post goes live on March 6th: gitlab-com/www-gitlab-com!19219 (merged). As long as this doesn't end up in 11.8 this should work out fine.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/52877

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• [-] Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [-] Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• [-] Security reports checked/validated by reviewer