Make eth_accounts return impersonated accounts

[MartinquaXD]

Motivation

Fix for #5732 (see that issue for detailed context).

Solution

Added CheatsManager::impersonated_accounts() to allow fetching impersonated accounts from EthApi.
Collected relevant addresses in EthApi::accounts() into a HashSet to avoid returning duplicate accounts when a user tries to impersonate an account that is already owned by the node.
Extended the can_impersonate_account unit test to assert that the impersonated account gets returned by the eth_accounts handler.

[mattsse]

supportive

[mattsse]

I'd like to keep a consistent order here and still use vec, but use an additional hashmap to filter unique addresses,
accounts.extend(signer.accounts().filter(|acc| unique.insert(*acc)))

[MartinquaXD]

I implemented what I think you suggested: bffe055
However, this ended up a bit verbose because the filter needs to be applied to both iterators. If you don't care about listing the original accounts first and the impersonated accounts later an alternative implementation could also be this:

        let mut accounts = Vec::new();
        for signer in self.signers.iter() {
            accounts.append(&mut signer.accounts());
        }
        accounts.extend(self.backend.cheats().impersonated_accounts());
        accounts.sort_unstable();
        accounts.dedup();

LMK what you prefer.

[MartinquaXD]

I'd like to keep a consistent order here

Actually since impersonated_accounts returns a HashMap the iterator produced by that already has an unstable order if you are impersonating > 1 accounts.