chore(objectstorage): add retry on 429 for objectstorage control plane

[h3adex]

Description

Adds retry for object storage control plane API

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]

Test Code:

// Single bucket
resource "stackit_objectstorage_bucket" "test" {
  project_id = var.stackit_project_id
  name       = "rate-limit-test"
}

# Read the same bucket 200 times via separate datasource instances.
# for_each over a set of indices gives 200 independent Read calls,
# each hitting the Object Storage API enough to trigger the 60 req/min limit
# and exercise the retry + jitter logic without creating 100 real buckets.
data "stackit_objectstorage_bucket" "test" {
  for_each = toset([for i in range(200) : tostring(i)])

  project_id = stackit_objectstorage_bucket.test.project_id
  name       = stackit_objectstorage_bucket.test.name
  region     = stackit_objectstorage_bucket.test.region
}

Current upstream build (Error is wrong displayed it is 429 and not 404). Also fixed that:

With new build trying to read 200 buckets: