Draft: Add pg_upgrade --run-specific-check and --list-checks

[Chibin]

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Some of the pg_upgrade checks may take a long time to run, so if a user wants to address a particular issue, they will likely want to run a specific check to validate that they've addressed the issue. Add the initial implementation of running a list of specific checks for pg_upgrade. Also added pg_upgrade --list-checks to know which checks are available to run individually.

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• [ ] Add tests for the change
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