DNS Challenge: no support for mTLS client certificates (e.g. reg.ru now requires it)

[FerrumLogic]

Problem

Some DNS providers now require mutual TLS (mTLS / client certificate) authentication on their API in addition to username/token.

Concrete example: reg.ru recently started requiring a client certificate for its API. Requests without a client cert are rejected with:

{"error_code": "NO_CERTIFICATE", "error_text": "The certificate not found"}

This breaks Let's Encrypt DNS-01 challenges and certificate auto-renewal for users whose DNS is hosted on such providers.

Impact

• Certificate issuance/renewal via the DNS Challenge fails.
• Users cannot configure a client certificate (cert + key) anywhere in the NPM UI for DNS challenge credentials — only username/password-style fields are exposed.

Suggested solution

Allow DNS challenge configuration to include an optional client certificate (cert + key, or a combined PEM) that is passed to the ACME client (lego / certbot DNS plugin) when talking to the provider API.

Workaround (for reference)

For the certbot-based path, this can be worked around by patching the bundled certbot-dns-regru plugin to send a client cert, but it is lost on container recreation and is not exposed via the UI. See certbot-dns-regru PR: shadowpercifal/certbot-dns-regru#1

[shadowpercifal]

shadowpercifal/certbot-dns-regru#1 has been tested and merged, see #4974 for nginx-proxy-manager image builds that use certbot-dns-regru package