feat(expr): implement pg_get_keywords() and pg_keywords

[wangrunji0408]

Signed-off-by: Runji Wang [email protected]I hereby agree to the terms of the RisingWave Labs, Inc. Contributor License Agreement.

What's changed and what's your intention?

resolve #17023

This PR implements pg_get_keywords() function and pg_keywords system catalog.
However, the implementation is not entirely correct. There are many rows in the output that have a different category than postgres. I don't fully understand how to map our reserved keyword tables to the category defined by PG. But if it's just to resolve the problem mentioned in #17023, maybe non-empty output is enough.

Checklist

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Release note

Support pg_get_keywords() function and pg_keywords system catalog.

[neverchanje]

Thanks. I tested locally and it worked.