In get_custody_groups, don't skip 0 value

[jtraglia]

This PR does the following:

• If there's an overflow, don't skip the zero value.
  • Previously this would set it to zero THEN increment.
  • Instead we should set to zero OR increment.
• Use more strict type annotations for explicit clarity.
• Remove unnecessary uint256 cast.
• When there's an overflow, cast zero to uint256 not NodeID.

Thanks to @tersec for pointing this out on Discord.

is it intentional that https://github.com/ethereum/consensus-specs/blob/dev/specs/fulu/das-core.md#get_custody_groups in the name of "Overflow prevention" skips the zero NodeID? That's not just overflow protection, that's conceptually, the NodeID sequence it iterates through is 115792089237316195423570985008687907853269984665640564039457584007913129639934, 115792089237316195423570985008687907853269984665640564039457584007913129639935, not 0, 1, 2, ...

[ppopth]

LGTM