IVault.VaultActionData no longer needs accountIds to be an array

[stevieraykatz]

We initially decided to make the VaultActionData.accountIds an array so that cross-chain harvest calls could be made. In #60 we decided that we can let a native contract on each chain call harvest which then means that the harvest endpoint doesn't necessarily need to comply with the cross-chain endpoint format.

This change will save gas costs on each run as we won't need to access an array element nor create 1-element arrays when calling invest or redeem