Gpsanant/fix pending checkpoint case

[gpsanant]

The previous logic did not handle the case where there was pending checkpoint on the pod so some validators were proven withdrawn. This would lead to them not being active, which would have them not be counted in the previous regGwei nor in the sumPreviousBeaconBalancesGwei, but they would be in the podBalanceGwei.

This would lead to validators that are proven withdrawn in a pending checkpoint simply not existing in the "previous balance".

In order to avoid this case, we simply calculate the previous balance only on the execution layer by asking the delegation manager for the actively restaked shares and the shares in the withdrawal queue.