get_orders() API doesn't allow getting "all" orders

[kaspar030]

The official API (and the docs copied from there) allow specifying "all" orders, but the code only allows passing Option<OrderStatus>:

https://docs.rs/coinbase-pro-rs/0.6.9/src/coinbase_pro_rs/private.rs.html#357-372

Not sure if the parameter would need it's own enum, or allow a list of order statuses, or if "all" should be the default. As is it is a bit clumsy to get "all", as that would require looping over the enum.