Add dropdown and feature flag [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This is the first step in adding the dropdown for the graph switcher and eventually making it possible for us to display the pipeline graph by needs as well as by stage.

Followups include !56974 (closed), which adds the graph and another which will persist the choice in the dropdown by user. (See #298930 (closed))

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this is behind a flag.
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Related to #298930 (closed)