See comments for prototype code links. This ticket includes adding relevant (golden) tests for multiplanning distinct(), and making sure that when the feature flag is enabled:

• We're still generating distinct scan plans as before, but now from all eligible indexes.
• Multiplanning generates/picks alternate index scans when we have few/no duplicates; otherwise favoring distinct scan as before.
• We update existing tests that may be broken by this behaviour when the feature flag is enabled.