feat(account): add billing summary

[MarinX]

This PR adds billing summary support to the account service.
Follows the API /account/billing/summary/{year-month} endpoint as documented in the account section.

Notes:

• The BillingCategory and BillingResourceGroup structs handle the nested JSON structure where categories contain both a total_amount and a nested object (e.g., servers.server, managed_databases.managed_database). If there is a better approach, let me know.
• Some billing categories like storages and networks are not included as they require specific API response examples. If you can provide with that details, I can implement.

[kangasta]

Thank you for the pull request!

Here's some example data for networks and storages from a test account:

{
  "currency": "EUR",
  "networks": {
    "ipv4_address": {
      "resources": [
        {
          "amount": 0.00438,
          "details": [
            {
              "amount": 0.00438,
              "hours": 1,
              "zone": "fi-hel1"
            }
          ],
          "hours": 1,
          "resource_id": "10.1.2.3"
        }
      ],
      "total_amount": 0.00438
    },
    "total_amount": 0.00438
  },
  "storages": {
    "backup": {
      "resources": [
        {
          "amount": 0.00078,
          "details": [
            {
              "amount": 0.00078,
              "hours": 1,
              "size": 10,
              "zone": "fi-hel1"
            }
          ],
          "hours": 1,
          "resource_id": "01e182d2-eb12-40dd-b473-afdff6300d9b"
        }
      ],
      "total_amount": 0.00078
    },
    "storage": {
      "resources": [
        {
          "amount": 0.00775,
          "details": [
            {
              "amount": 0.00775,
              "hours": 1,
              "labels": [
                {
                  "key": "_os_architecture",
                  "value": "64"
                },
                {
                  "key": "_os_brand_name",
                  "value": "Kubernetes"
                },
                {
                  "key": "_os_brand_version",
                  "value": "1.30"
                },
                {
                  "key": "_os_main_category",
                  "value": "Linux"
                },
                {
                  "key": "_os_storage",
                  "value": "01000000-0000-4000-8000-000020070100"
                },
                {
                  "key": "_os_type",
                  "value": "debian"
                },
                {
                  "key": "_template_uuid",
                  "value": "01000000-0000-4000-8000-000160060100"
                }
              ],
              "size": 25,
              "zone": "fi-hel2"
            }
          ],
          "hours": 1,
          "resource_id": "01bc075d-6bfb-4827-9e51-e49734017614"
        }
      ],
      "total_amount": 0.00775
    },
    "template": {
      "resources": [
        {
          "amount": 0.0031,
          "details": [
            {
              "amount": 0.0031,
              "hours": 1,
              "size": 10,
              "zone": "fi-hel1"
            }
          ],
          "hours": 1,
          "resource_id": "01ee2ceb-882f-4eb0-84a0-968f66449047"
        }
      ],
      "total_amount": 0.0031
    },
    "total_amount": 0.01163
  },
  "total_amount": 0.01601
}

There could also be bandwidth key under networks, but I don't have testdata for that at the moment.

[MarinX]

Perfect. Thanks for the sample JSON.
Updated with additional billing categories.