ext/pdo_pgsql: Retrieve the memory usage of the query result resource (#14260)

`getAttribute()` can now retrieve the memory usage of query results.
`PDO::PGSQL_ATTR_RESULT_MEMORY_SIZE` w

ext/pdo_pgsql: Retrieve the memory usage of the query result resource (#14260)

`getAttribute()` can now retrieve the memory usage of query results.
`PDO::PGSQL_ATTR_RESULT_MEMORY_SIZE` was added for this feature.

closes #14260

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