Add Cop against dynamic FF keys

Description of the proposal

Add Cop against dynamic FF keys

Feature flag calls should be static as dynamic keys make it hard to be detect usage.

Additional reason: Dynamic feature flags that return that state of an FF over the network makes removing feature flags even harder, makes it more like a setting. => https://gitlab.com/gitlab-org/gitlab/-/issues/522906

Check-list

• Make sure this MR enables a static analysis check rule for new usage but ignores current offenses.
• Mention this proposal in the relevant Slack channels (e.g. #development, #backend, #frontend).
• [-] If there is a choice to make between two potential styles, set up an emoji vote in the MR:
  • CHOICE_A: 🅰️
  • CHOICE_B: 🅱️
  • Vote for both choices, so they are visible to others.
• The MR doesn't have significant objections, and is getting a majority of 👍 vs 👎 (remember that we don't need to reach a consensus).
• (If applicable) One style is getting a majority of vote (compared to the other choice).
• (If applicable) Update the MR with the chosen style.
• Create a follow-up issue to fix the current offenses as a separate iteration: Fix current offenses for Gitlab/FeatureFlagKeyD... (#531374), and #531375 (closed)
• Follow the review process as usual.
• Once approved and merged by a maintainer, mention it again:
  • In the relevant Slack channels (e.g. #development, #backend, #frontend).
  • (Optional depending on the impact of the change) In the Engineering Week in Review.

/cc @gitlab-org/maintainers/rails-backend