Add SchedulePlanChangeRequest, make CustomerCostParamsFilter pub

[aarashy]

Adds the API for updating a customer subscription in place.

Also makes CustomerCostParamsFilter pub

[arusahni]

Thanks for filing this PR @aarashy! Could you elaborate upon the need for changing the visibility of CustomerCostParamsFilter? This is intentionally not exposed outside of the crate as we use the builder pattern for parameter collections. So, instead of doing something like:

get_customer_costs(&customer_id, &CustomerCostParams { filter: CustomerCostParamsFilter { view_mode: CostViewMode::Periodic, ..Default::default() } })

clients can instead invoke this as

get_customer_costs(&customer_id, &CustomerCostParams::default().view_mode(CostViewMode::Periodic))