Support for Manual DNS-01 Challenge (Custom _acme-challenge TXT Record)

Hello,
Currently, Nginx Proxy Manager supports issuing SSL certificates via Let's Encrypt using the HTTP challenge and a limited number of DNS providers (via API access). However, it would be very useful to have the option to use the DNS-01 challenge manually, by allowing the user to manually add the required _acme-challenge TXT record in their DNS zone.

This feature is especially important when:

The DNS provider is not supported by NPM;

The user doesn't want to share API credentials with a third-party system;

The domain is hosted in a restricted or enterprise DNS environment.

Many panels like FastPanel already support this feature: during certificate issuance, the system displays the required TXT record, waits for the user to add it, and then proceeds with verification.

Proposed flow:

User selects "Manual DNS Challenge" option.

NPM generates the required TXT record.

NPM pauses and displays the _acme-challenge name and value.

User confirms once the record is added.

NPM verifies and obtains the certificate.

This would make NPM more flexible and usable across a wider range of environments.

Thank you for considering this!

The issue names no files or tests. Start by tracing NPM's existing HTTP-01 and DNS-provider certificate issuance flows, then define how the manual DNS-01 state is represented and surfaced. Done means users can view the _acme-challenge TXT name and value, confirm its creation, and complete verification and certificate issuance.