eth/downloader, core/types: take withdrawals-size into account in downloader queue

[psogv0308]

This PR addresses an issue where the size of the downloader queue was not accurately measured. Previously, the size of withdrawals was not included in the queue size calculation, leading to an accumulation of more items than originally intended. This simple fix ensures that withdrawals are now considered, providing an accurate measurement of the queue size.

[fjl]

Please avoid the use of package unsafe here. Since withdrawals are defined by the protocol and will never change, we can just hard-code the returned size here.

[psogv0308]

var withdrawalSize = common.StorageSize(reflect.TypeOf(Withdrawal{}).Size())

func (w Withdrawal) Size() common.StorageSize {
	return common.StorageSize(withdrawalSize)
}

What do think about handling it like this, similar to the code in other parts?

[holiman]

Since withdrawals are constant size, perhaps put Size on Withdrawals instead, and do

size += common.StorageSize(result.Withdrawals.Size())

(yes, make the conversion to StorageSize here, not in the Size-function)

Then the change in types boils down to

var withdrawalSize = int(reflect.TypeOf(Withdrawal{}).Size())

func (s Withdrawals) Size() int {
	return withdrawalSize * len(s)
}

[holiman]

LGTM