STACKITRCO-187 - Add option iaas API param agent

[aeter]

Adds a terraform stackit_server option for the iaas ( create server ) API param: "agent": {"provisioned": true}

ref STACKITRCO-187

ref: stackitcloud/stackit-cli#1213

---

Tests:

• ran make fmt, make generate-docs

• ran unit tests

[~/terraform-provider-stackit] go test stackit/internal/services/iaas/server/*
ok      command-line-arguments  15.005s
[~/terraform-provider-stackit] make test
...
ok      github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/iaas/server       15.006s  coverage: 33.0% of statements
...
[~/terraform-provider-stackit]

• Tested: with a locally-configured terraform - tested by adding, changing, deleting agent-related parts of the below main.tf ** Tested without providing agent inside main.tf (the result had "agent" = null /* object */)

** Tested with setting agent = { provisioning = true } (and then ran the stackit-cli command for checking if the agent was created successfully and able to run commands, stackit -y server command create --server-id=3fdac6ea-3885-441c-b473-bc94ca570ca8 --project-id=c904f41c-2f8c-4edb-b966-e87d65f10b64 --template-name=RunShellScript --params script='echo hello').

** Tested when setting agent = { provisioning = true } and then set it to false - verified the server was deleted and recreated again with the new value.

[~] cat main.tf
provider "stackit" {
  # Configuration options
  service_account_key_path = "/home/debian/terraform_dev/.terraform_key.json"
  default_region = "eu01"
}

resource "stackit_network_interface" "server_nic" {
  project_id = "c904f41c-2f8c-4edb-b966-e87d65f10b64"
  network_id = "97c5dde4-cb9d-49b8-be55-9cdf0c3795e1"
}

resource "stackit_server" "myserver1" {
    project_id = "c904f41c-2f8c-4edb-b966-e87d65f10b64"
    name = "terraformtestserver1"
    boot_volume = {
        size = 64
        source_type = "image"
        source_id = "21466190-b904-4267-8bf3-1be4323f4ffb"
        delete_on_termination = true
    }
    availability_zone = "eu01-1"
    agent = {
        provisioned = true
    }
    machine_type = "t1.1"
    network_interfaces = [ stackit_network_interface.server_nic.network_interface_id ]
}

data "stackit_server" "myserver1_data" {
   project_id = "c904f41c-2f8c-4edb-b966-e87d65f10b64"
   server_id  = stackit_server.myserver1.server_id
}

output "server_info_from_data" {
  value = data.stackit_server.myserver1_data
}

[~] terraform apply -auto-approve  ## this is without `agent` set in the config
...
server_info_from_data = {
  "affinity_group" = tostring(null)
  "agent" = null /* object */
  "availability_zone" = "eu01-1"
  ...
}
...

[~] terraform apply -auto-approve  ## this is with `agent = { provisioned = true }` set in the config
...
server_info_from_data = {
  "affinity_group" = tostring(null)
  "agent" = {
    "provisioned" = true
  }
  "availability_zone" = "eu01-1"
  "boot_volume" = {
    "delete_on_termination" = true
    "id" = "673021e5-2a90-4482-8ffa-e0485c7588bd"
  }
  ...
}

Description

relates to STACKITRCO-187

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Acceptance tests got implemented or updated (see e.g. here)
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)