Add custom accounts to Anvil genesis config

[re-gius]

alloc genesis customization for foundry-polkadot with unit tests.

This covers both EOA and contract accounts.

[iulianbarbu]

This looks great! Looking forward to the tests 💪

[alindima]

We need some coordination with #351, because we need to add those generated accounts to the genesis storage as well

[iulianbarbu]

We need some coordination with #351, because we need to add those generated accounts to the genesis storage as well

Maybe we can merge this first, and then #351, accounting for what's here.