Fix some issues in new random execution requests

[jtraglia]

• Rename methods so the request type isn't plural.
  • For example, get_random_deposits_requests to get_random_deposit_requests.
• Set DepositRequest.index to rng.randrange(0, 2**64).
  • In Electra, index doesn't exist in deposit data.
  • I would have done rng.randint(0, UINT64_MAX) but this is an alias of randrange(a, b+1) and we cannot increment the max uint64 value again. Kind of a weird edge case.
• Delete some whitespace & fix some organization nits.

[jtraglia]

Going to merge now to fix test vectors.