feat(cloudflare): add support for MX records

[arthlr]

trafficstars

Hi there! 👋

Description

This PR introduces support for MX records for the Cloudflare provider.

• Cloudflare can now manage MX records via the DNSEndpoint CRD.
• Adds logic to parse MX records and extract priority and mail host
• Extends unit tests to validate proper MX record creation and priority
• Updates the documentation to reflect support for cloudflare in MX record management.

Fixes https://github.com/kubernetes-sigs/external-dns/issues/5282

Checklist

• [x] Unit tests updated
• [x] End user documentation updated (docs/sources/mx-record.md)

Thanks for checking this out! 🙏 Looking forward to your feedback — happy to make any changes if needed.