Don't accept remote blocks without mixHash fields

[fvictorio]

In #516 we changed the code to accept remote blocks without mixHash fields. If the field is not present, the default (zero) value is used. We did this to accommodate EVM-compatible chains that don't have this field.

We should re-visit this after we have proper support for alternative chain types. Mainnet-like chains should still expect this field to be present, but other chain types could make it optional.

[alcuadrado]

I think the title of the issue makes an implicit assumption that we'd just reject working with networks that we don't fully understand/support. This is contrary to how Hardhat Network has been working so far.

[fvictorio]

Yes, we need to discuss this more.

@Wodann suggested something I quite like: besides support for Ethereum Mainnet and Optimism, we could have a "Fallback" chain type which is more permissive. In that case, a mix hash wouldn't be accepted in "Ethereum Mainnet" mode, but it would in fallback mode.

[fvictorio]

Closing this because it's pretty clear that we'll have that fallback chain type, which would make this a non-issue.