Allow querying all granted allowances to a spender

[abefernan]

I propose implementing a QueryMsg::AllGrantedAllowances similar to QueryMsg::AllAllowances that allows querying all of the allowances that have been made to a spender's address, with this API:

AllGrantedAllowances {
    spender: String,
    start_after: Option<String>,
    limit: Option<u32>,
}