feat: add cidr_list attribute to stackit_public_ip_ranges datasource

[h3adex]

Description

This PR makes working with the datasource easier. Current way to work with it to enable stackit acls for stackit public ranges is this way:

data "stackit_public_ip_ranges" "this" {}

# old way
locals {
  stackit_services_cidrs = [for ip in data.stackit_public_ip_ranges.this.public_ip_ranges : ip.cidr]
}

# improved through pr
data.stackit_public_ip_ranges.this.cidr_list

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)

[rubenhoenle]

@h3adex Thanks for the contribution! Can you maybe also adjust the acc tests for this?

[h3adex]

E2E Tests

[github-actions]

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