make QueuedQuery.Fn property public, closes #1878

[pashagolub]

This commit fixes the overlook of the #1886 where QueuedQuery.SQL and QueuedQuery.Arguments properties were exposed.

And if for SQL and Arguments we had an opportunity to use reflection, for QueuedQuery.fn it's impossible.

One more benefit exposing QueuedQuery.Fn for developers is the ability to execute batched functions without knowing the kind of a queued query and mandatory calling BatchResults.Close(), e.g.

batch := &pgx.Batch{}
batch.Queue("select func42($1)", 42).QueryRow(...)
batch.Queue("select * from foo").Query(...)
batch.Queue("insert").Exec(...)
...
batch.Queue("truncate foo").Exec(...)

br = conn.SendBatch(ctx, batch)
// check first two queued queries
err := batch.QueuedQueries[0].Fn()
err = errors.Join(err, batch.QueuedQueries[1].Fn())
// let BatchResults execute all that left and close 
err = errors.Join(err, br.Close())
if err != nil {
	fmt.Printf("SendBatch error: %v", err)
	return
}

[pashagolub]

When this PR will be merged, one will be able to test Batches with pgxmock:

func TestBatch(t *testing.T) {
	t.Parallel()
	mock, _ := NewConn()
	a := assert.New(t)

	// define our expectations
	eb := mock.ExpectBatch()
	eb.ExpectQuery("select").WillReturnRows(NewRows([]string{"sum"}).AddRow(2))
	eb.ExpectExec("update").WithArgs(true, 1).WillReturnResult(NewResult("UPDATE", 1))

	// run the test
	batch := &pgx.Batch{}
	batch.Queue("select 1 + 1").QueryRow(func(row pgx.Row) error {
		var n int32
		return row.Scan(&n)
	})
	batch.Queue("update users set active = $1 where id = $2", true, 1).Exec(func(ct pgconn.CommandTag) (err error) {
		if ct.RowsAffected() != 1 {
			err = errors.New("expected 1 row to be affected")
		}
		return
	})

	err := mock.SendBatch(ctx, batch).Close()
	a.NoError(err)
	a.NoError(mock.ExpectationsWereMet())
}