Cop for gem fetched from a git source

What does this MR do?

Alert if Gemfile contains gem with git or github as parameter, for example:

gem 'xyzgem', git: 'git@gitlab.com:xyz/xyzgem.git', tag: '1.2.3'
gem 'json', github: 'flori/json', ref: 'v1.8.2'

Are there points in the code the reviewer needs to double check?

Yes

Why was this MR needed?

To reduce external dependencies not from the RubyGems index, which help to minimise the build times

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #27311 (closed).