stackit_loadbalancer: support HTTP health checks, alt_port and labels #1825

Problem description

The Network Load Balancer API supports HTTP health checks, an alternative health check port and labels, but stackit_loadbalancer doesn't expose them:

• targetPools[].activeHealthCheck.altPort and targetPools[].activeHealthCheck.httpHealthChecks (path, okStatuses) — the provider only exposes healthy_threshold, interval, interval_jitter, timeout and unhealthy_threshold, so every NLB health check is a plain TCP check on the target port.
• labels on the load balancer (create and update payload).

All of these are already available in stackit-sdk-go/services/loadbalancer/v2api (ActiveHealthCheck.AltPort, ActiveHealthCheck.HttpHealthChecks, CreateLoadBalancerPayload.Labels / UpdateLoadBalancerPayload.Labels), and stackit_application_load_balancer already supports the same health check options (alt_port, http_health_checks.path, http_health_checks.ok_status) and labels.

Proposed solution

Add the same attributes to stackit_loadbalancer (resource and data source), following the ALB implementation:

resource "stackit_loadbalancer" "example" {
  # ...
  labels = {
    env = "dev"
  }

  target_pools = [{
    name        = "example-pool"
    target_port = 80
    targets     = [{ display_name = "example", ip = "192.168.0.10" }]

    active_health_check = {
      healthy_threshold   = 3
      interval            = "10s"
      interval_jitter     = "3s"
      timeout             = "3s"
      unhealthy_threshold = 3
      alt_port            = 8080
      http_health_checks = {
        path      = "/healthz"
        ok_status = ["200"]
      }
    }
  }]
}

I'd like to implement this and would split it into two PRs: health check options (alt_port, http_health_checks) first, then labels.