pdns: support multiple API endpoints

Welcome

• Yes, I'm using a binary release or the library within the two latest releases.
• Yes, I've searched for similar issues on GitHub and didn't find any.

Detailed Description

I would like to add support for multiple PowerDNS API endpoints to the existing pdns DNS provider.

Use case: Some PowerDNS deployments run several independent authoritative/master nodes. During DNS-01 validation, the TXT record should be created on all authoritative nodes, not only on a single API endpoint.

Proposed approach:

• Keep existing PDNS_API_URL and PDNS_API_KEY behavior unchanged.
• Add optional PDNS_API_URLS and PDNS_API_KEYS.
• If PDNS_API_URLS is set, the provider creates/cleans TXT records on every configured PowerDNS API endpoint.
• PDNS_API_KEYS can either contain one key reused for all URLs or one key per URL.
• Scope is intentionally limited to PowerDNS provider only, not a generic multi-provider abstraction.

I can prepare a focused PR with tests and documentation if this approach is acceptable.

Effective version of lego

latest

How do you use lego?

Binary, Library

In which context are you using lego?

Professional use

Supporting the Maintainers

• Yes, I find this project useful, and I appreciate the work done by the maintainers.
• Yes, I have starred this repository to help others find it.
• Yes, I have sponsored this project to keep the project stable and maintained.

Confirmation

• Yes, I've included all the information above (version, usage, etc.).