Allow custom ACME server address for SSL certificate requests

Is your feature request related to a problem? Please describe.

So we are running our own certificate authority, which has a custom endpoint for issuing SSL certificates using ACME. For now when we try to issue an SSL certificate, it takes a lot of manual steps to request the certificate and import it into Nginx Proxy Manager. It would be useful for the already implemented Let's Encrypt feature to have an additional (normally hidden) argument to provide your own ACME server.

Describe the solution you'd like

It would be useful for the Nginx Proxy Manager 'Request a new SSL Certificate' feature to maybe have a boolean called 'Advanced' which on press would show an additional input field called 'Server' where one could provide their own ACME endpoint.

Describe alternatives you've considered

We are at the moment able to shut down nginx proxy manager, then run certbot with --server https://ca.example.org/acme/acme/directory to generate SSL certificates, copy them over to our own host, start up Proxy manager and then upload the certs in SSL Certificates > Add SSL Certificate > custom, but this takes a lot of additional steps.

Additional context

No files or tests are named in the issue. Start by tracing the existing “Request a new SSL Certificate” and Let's Encrypt flow; done means an Advanced option accepts a custom ACME server and requests the certificate through it while the existing flow still works.