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, make lint

* 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, by adding, changing, deleting `agent`-related parts of a tf config.

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)

[github-actions]

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[cgoetz-inovex]

lgtm besides the docs

[aeter]

please review.

I've updated the code as requested - added provisioning_policy. Retested. I ran the acc tests, they failed with unrelated to my code failures.

---

acceptance tests

[~/terraform-provider-stackit] TF_ACC=1 TF_ACC_PROJECT_ID=c904f41c-2f8c-4edb-b966-e87d65f10b64 STACKIT_SERVICE_ACCOUNT_KEY_PATH=~/terraform_dev/.terraform_key.json TF_ACC_REGION=eu01 go test -timeout=60m -v ./stackit/internal/services/iaas/... -run "TestAccServer(Min|Max)"

A server created without sending 'provisioning' flag via the CLI - and then imported into terraform:

[~/terraform_dev] STACKIT_TF_ENABLE_BETA_RESOURCES=true TF_LOG_PROVIDER=DEBUG terraform import stackit_server.imported_server c904f41c-2f8c-4edb-b966-e87d65f10b64,eu01,7359173a-b476-4c3c-9cd4-35c7984114e2
[~/terraform_dev] terraform show
Outputs:

imported_server = {
   agent             = {
       provisioning_policy = "INHERIT"
   }
   id                = "c904f41c-2f8c-4edb-b966-e87d65f10b64,eu01,7359173a-b476-4c3c-9cd4-35c7984114e2"
}

A server created in terraform with defaults (no 'agent' block):

server_info_from_data = {
 "agent" = {
   "provisioned" = tobool(null)
   "provisioning_policy" = "INHERIT"
 }
 "id" = "c904f41c-2f8c-4edb-b966-e87d65f10b64,eu01,b4e69af2-a8a6-407b-81e5-b32df3041719"

A server created in terraform like this. Note the ALWAYS->INHERIT in datasource, which can/should be fixed when the API is fixed one day:

//------------ main.tf
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 = "nackovterraform1"
   boot_volume = {
       size = 64
       source_type = "image"
       source_id = "21466190-b904-4267-8bf3-1be4323f4ffb"
       delete_on_termination = true
   }
   availability_zone = "eu01-1"
   agent = {
       provisioning_policy = "ALWAYS"
   }
   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
}

output "server_info_from_resource" {
  value = stackit_server.myserver1.agent
}
//------------

####### output from make apply
server_info_from_data = {
 "affinity_group" = tostring(null)
 "agent" = {
   "provisioned" = true
   "provisioning_policy" = "INHERIT"
 }
}
server_info_from_resource = {
  "provisioned" = true
  "provisioning_policy" = "ALWAYS"
}

[aeter]

updated per review