add seed index param for regenerating keys

[jenpaff]

When generating keys with an existing mnemonic, it's important to specify the seed index of the given key that is being regenerated. This is such that we can stay aligned with the EIP-2334 format (see: https://eips.ethereum.org/EIPS/eip-2334), the seed index represents the account which is necessary to derive the withdrawal and signing keys.

We add a parameter such that a user can specify the start index from which the keys should be generated

see implementation in staking-deposit-cli

# Set path as EIP-2334 format
# https://eips.ethereum.org/EIPS/eip-2334
purpose = '12381'
coin_type = '3600'
account = str(index)
withdrawal_key_path = f'm/{purpose}/{coin_type}/{account}/0'
self.signing_key_path = f'{withdrawal_key_path}/0'

Tasks:

• [x] add validator start index param
• [x] update key store unit tests
• [x] add e2e tests