Observability: add resources for HTTP and certificate checks #1829

Problem description

The Observability API offers health checks per instance (/v1/projects/{projectId}/instances/{instanceId}/http-checks, /cert-checks, and also ping, network, mysql, postgresql, mongodb, redis, rabbitmq and elasticsearch checks), but the provider has no resources for them. Today they can only be set up in the portal or through the API, so a monitoring setup managed with Terraform (instance, alert config, scrape configs) is missing its endpoint and certificate checks.

The operations are available in stackit-sdk-go/services/observability/v1api (e.g. CreateHttpCheck, ListHttpChecks, DeleteHttpCheck, CreateCertCheck, ListCertChecks, DeleteCertCheck).

Proposed solution

New resources, starting with the two most common checks:

resource "stackit_observability_http_check" "example" {
  project_id  = var.project_id
  instance_id = stackit_observability_instance.example.instance_id
  url         = "https://www.example.com/healthz"
}

resource "stackit_observability_cert_check" "example" {
  project_id  = var.project_id
  instance_id = stackit_observability_instance.example.instance_id
  source      = "tcp://www.example.com:443"
}

The API has no update or single-get operation for checks, so all attributes would force a replacement and Read would look the check up by ID in the list response.

I'd like to implement this, one PR per resource (HTTP check first, then cert check). The other check types could follow the same pattern later.