Implement new beacon APIs for pending deposits/partial withdrawals

[michaelsproul]

Two new informational APIs have been added to the beacon API spec in this PR:

• added pending deposits and pending partial withdrawals accessors ethereum/beacon-APIs#500

We should implement them. It should be straight-forward. We can error with a 400 if a pre-Electra state is requested.

[syjn99]

Can I tackle this?

[michaelsproul]

@syjn99 Sure!

[syjn99]

@michaelsproul Hi, I just opened the draft PR for addressing this issue, and wonder if I'm missing sth or doing right. (I didn't run unit tests yet since it is blocked by #7005.) Could you check?

[michaelsproul]

run the unit tests with --release and they will pass