The public key bytes to be used as the account's alias. The public key bytes are the result of serializing a protobuf Key message for any primitive key type. Currently only primitive key bytes are supported as an alias (ThresholdKey, KeyList, ContractID, and delegatable_contract_id are not supported). At most one account can ever have a given alias and it is used for account creation if it was automatically created using a crypto transfer. It will be null if an account is created normally. It is immutable once it is set for an account. If a transaction auto-creates the account, any further transfers to that alias will simply be deposited in that account, without creating anything, and with no creation fee being charged.