feat(pgwire): support struct type in extended mode

[fuyufjh]

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What's changed and what's your intention?

Closes #7949.

sqllogictest-rs doesn't support struct type yet, so I added a Python test in e2e-test.

References:

• Postgres code src/backend/utils/adt/rowtypes.c
• Pseudo-type of struct & array of struct: src/include/catalog/pg_type.dat

Checklist

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