Add support for resource billing summary APIs

[vmj]

There's a new API

• GET /1.3/account/resource_billing_summary/RESOURCE_ID/YYYY-MM

where RESOURCE_ID can be a server or storage UUID or an IP address.

https://developers.upcloud.com/1.3/3-accounts/#get-monthly-resource-billing-summary

This issue is separate from #2 but the implied (new) Billing resource will be a mess.

[vmj]

This seems to be available in 1.2 API, too.

[vmj]

The response has the following structure:

• Billing (new resource)
  • currency: String
  • totalAmount: BigDecimal
  • details: Details (new resource)
    • type: String
    • resourceId: String
    • hours: Integer
    • ... some type specific fields ...
    • plan: String (for server type)
    • size: Integer (for storage type)
  • dailySums: DailySums (new resource)
    • ... all fields are named "YYYY_MM_DD", type BigDecimal ...

That's ugly ☹️