ext/pgsql: adding pg_close_stmt.

up to postgresql 17, when done with a prepared statement, we could
release it with DEALLOCATE sql command which is fine ; until we want
to implement

ext/pgsql: adding pg_close_stmt.

up to postgresql 17, when done with a prepared statement, we could
release it with DEALLOCATE sql command which is fine ; until we want
to implement a cache solution based on statement ids.

Since PostgreSQL 17, PQclosePrepared uses internally the `close` protocol
allowing to reuse the statement name while still freeing it.
Since the close protocol implementation had been added on libpq within
this release, no way to reimplement it.

close GH-14584

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