Add support for monthly billing summary API

[vmj]

There are two new APIs:

• GET /1.3/account/billing_summary/YYYY-MM
• GET /1.3/account/billing_summary/YYYY-MM/detailed

See if they require one or two methods in UpCloudApi trait.

Also, check how they parse and if they conflict with any of the resources.

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

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

This is separate from #3 but the implied (new) Billing resource will be a mess.

[vmj]

These seem to be available in 1.2 API, too.

[vmj]

The monthly billing summary response has the following structure:

• Billing (new resource)
  • currency: String
  • totalAmount: BigDecimal
  • servers: Servers (new resource)
    • totalAmount: BigDecimal
    • server: Server
      • totalAmount: BigDecimal (new property in Server)
  • storages: Storages (new resource)
    • totalAmount: BigDecimal
    • storage: Storage
      • totalAmount: BigDecimal (new property in Storage)

Zero amounts are not returned. Will need to add the network support when working with the networks in #6

[vmj]

The detailed monthly summary response has the following structure:

• Billing (new resource)
  • currency: String
  • totalAmount: BigDecimal
  • servers: Servers (new resource)
    • totalAmount: BigDecimal
    • server: Server
      • totalAmount: BigDecimal (new property in Server)
      • resources: List<Resources> (new property in Server, new resource)
        • resourceId: String
        • amount: BigDecimal
        • hours: Integer
        • firewall: String
        • plan: String
  • storages: Storages (new resource)
    • totalAmount: BigDecimal
    • storage: Storage
      • totalAmount: BigDecimal (new property in Storage)
      • resources: List<Resources> (new property in Storage, new resource)
        • resourceId: String
        • amount: BigDecimal
        • hours: Integer
        • size: Integer

The response does not adhere to the "list wrapper" convention. Hence "resources" ends up being whatever List of Maps -type the JSON implementation generates. This needs support.