known_engine_accounts: Vec -> LookupSet

[olga24912]

We need to do the next operations with known_engine_accounts:

• contains
• insert
• remove
  https://github.com/aurora-is-near/aurora-eth-connector/blob/master/eth-connector/src/lib.rs#L60

It looks like operations for the LookupSet, not Vec. On the other hand, usually this vector will only store one element. Therefore, it is possible using a vector here is more optimal. I was confused that we are using a regular vector, and not from near_sdk::collections

[mrLSD]

Refactored at #50