feat(resourcemanager): add folder resource/datasource

[h3adex]

Description

Implements full support for resource and data source for folders (STACKITTPR-213). Both are marked as beta.

Acceptance tests/Docs updated for:

stackit_resourcemanager_project  
stackit_resourcemanager_folder

stackit_resourcemanager_project improvements:

• Added attributes: created_at, updated_at
• Removed incorrect service_account_email documentation

Example configuration:

locals {
  org_id = "xxxx"
}

resource "stackit_resourcemanager_folder" "folder01" {
  name                = "test"
  owner_email         = "<your-mail>@stackit.cloud"
  parent_container_id = local.org_id
  labels = {
    "env" = "dev"
  }
}

data "stackit_resourcemanager_folder" "folder01" {
  container_id = stackit_resourcemanager_folder.folder01.container_id
}

# Projects can be added under folders. When deleting a project,
# it may remain invisible for up to 7 days.
# Folder deletion may fail during this period.
/*resource "stackit_resourcemanager_project" "project01" {
  name                = "proj-01-updated"
  owner_email         = "<your-mail>@stackit.cloud"
  parent_container_id = stackit_resourcemanager_folder.folder01.container_id
}*/

E2E test results in the comments.

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]

E2E Run

STACKIT_SERVICE_ACCOUNT_TOKEN=eyJ
TF_ACC=1
TF_ACC_PROJECT_ID=xxxx-xxxx-xxxx-xxxx
TF_ACC_REGION=eu01
TF_ACC_TEST_PROJECT_PARENT_UUID=xxxx-xxxx-xxxx-xxxx
TF_ACC_TEST_PROJECT_PARENT_CONTAINER_ID=test-company
TF_ACC_TEST_PROJECT_SERVICE_ACCOUNT_EMAIL=foo.bar@stackit.clouid