Renaming BurnResult -> WithdrawResult

[olga24912]

I propose to rename the BurnResult(https://github.com/aurora-is-near/eth-connector/blob/master/eth-custodian/contracts/EthCustodian.sol#L117) into WithdrawResult to be consistent with https://github.com/aurora-is-near/aurora-eth-connector/blob/master/eth-connector/src/lib.rs#L504

[sept-en]

@olga24912 the deployed EthCustodian is not a proxy, thus non-modifiable (without extra process of the migration). It's also important to emphasize that we are verifying the proof of burning the tokens on the NEAR side; that's why it's BurnResult.