feat(observability): implement cert and http checks

[h3adex]

Description

This PR introduces http and cert_check Terraform support. Once an HTTP or certificate check is created, you can query thanos within the observability stack for metrics (e.g http_response_result_code) and create alerts based on them.

APIs:
https://docs.api.stackit.cloud/documentation/argus/version/v1#tag/http-check
https://docs.api.stackit.cloud/documentation/argus/version/v1#tag/cert-check

E2E Tests

resource "stackit_observability_instance" "example" {
  project_id = var.stackit_project_id
  name       = "example-instance"
  plan_name  = "Observability-Medium-EU01"
  alert_config = null
}

resource "stackit_observability_http_check" "http_check" {
  project_id = var.stackit_project_id
  instance_id = stackit_observability_instance.example.instance_id
  url = "https://www.example.de"
}

data "stackit_observability_http_check" "test" {
  project_id = var.stackit_project_id
  instance_id = stackit_observability_instance.example.instance_id
  http_check_id = stackit_observability_http_check.http_check.http_check_id
}

resource "stackit_observability_http_check" "http_check_2" {
  project_id = var.stackit_project_id
  instance_id = stackit_observability_instance.example.instance_id
  url = "https://www.stackit.de"
}

resource "stackit_observability_cert_check" "cert_check" {
  project_id = var.stackit_project_id
  instance_id = stackit_observability_instance.example.instance_id
  source = "tcp://stackit.de:443"
}

data "stackit_observability_cert_check" "test" {
  project_id = var.stackit_project_id
  instance_id = stackit_observability_instance.example.instance_id
  cert_check_id = stackit_observability_cert_check.cert_check.cert_check_id
}

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)

[h3adex]

Sorted them alphabetically

[h3adex]

E2E Test Result:

[github-actions]

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