add clean cache function in pg_prewarm

[csuopen]

When we use pg_prewarm to cache a table into memory, we need to clean up the cache again when we need to see the results, so we added two functions:

• The function pg_clean_all_cache cleans up all the cache of the current database.
• The function pg_clean_rel_cache to clean up the cache of a table.

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[polardb-bot]

Hi @csuopen ~ Thanks for your contribution in this PR. ❤️

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[polardb-bot]

Hey @csuopen :

Congratulations~ 🎉 Your commit has passed all the checks. Please wait for further manual review.

[csuopen]

mistake something, PR should not be to ApsaraDB:POLARDB_11_STABLE, need to DEV branch. so close is, and create new pr to dev branch.