Feature Request: Generate .P12 Files

Feature Request:

I recently noticed there was no p12 files that could be generated through the nginx proxy GUI (from what I've seen). I was thinking the feature could be useful for certain self-hostings, such as Jellyfin or other websites that have the option to turn on "require https" feature.

Is your feature request related to a problem?

The feature is not related to a problem; however, I believe it adds another useful feature for when configuring a website and want to turn on require https. Having a easy path to provide that was generated from the key pair and root/issuing CAs would make it so that the .p12 file can be generated and saved with a password on the system running nginx-proxy-manager. Then the Server would be able to provide a single path to the website that it could use even on re-key's. The Password that will be used can be saved in the database or in a file within the config folder for nginx-proxy-manmager.

Possible Solution:

1. Add a checkbox or a new tab within nginx-proxy-manager to choose to generate a '.p12 ' after obtaining a certificate form lets encrypt.
2. If the Check-box is checked to create a pkcs12 a new optional box is provided displayed in the certificate request with the label "PKCS12 (p12) password:". Also Add a information hover over indicator that states, "This password will be used to generate a .p12 file, and will also be used for re-key of certificates."
   a. If the User omits the password and confirms to request the certificate from lets encrypt without the PKCS12 password a pop-up is displayed to let tell them the .p12 file will not be generated at this time, and can be manually generated at a later time.
   b. If the User Provides the password in the "PKCS12 (p12) password:" field then after the certificate is retrieved from lets encrypt, nginx-proxy-manager will run the OpenSSL command to generate the .p12 file.
   Examples:
   sudo openssl pkcs12 -export -out /path/to/pkcs12/Example.p12 -certpbe AES-256-CBC -keypbe AES-256-CBC -macalg SHA256 -inkey /path/to/privatekey/privkey.key -in /path/to/certificate/cert.pem -certfile /path/to/root/root.pem -passin /path/to/pkcs12/password/pkcs12.pass
   [or]
   sudo openssl pkcs12 -export -out /path/to/pkcs12/Example.p12 -certpbe AES-256-CBC -keypbe AES-256-CBC -macalg SHA256 -inkey /path/to/privatekey/privkey.key -in /path/to/certificate/cert.pem -certfile /path/to/root/root.pem -password $PKCS12_password
3. Re-run the command on rekey of certificates using the same password.
4. The Associated .p12 should be available in the same folder as the associated certificate and private key files.

Alternative Solution:

Add a button next to each SSL Certificate that says "Generate .p12/.pfx" and When the button is clicked it will run something like the OpenSSL command shown above. On successful generation of the file the button should show a check mark on the button to indicate its been generated. Maybe Grey out and disable the box for a limited period of time after creating the file?

Additional context

I think the feature might make a big quality of life improvement for self hosting certificates and SSL.

Start with the nginx proxy GUI’s certificate request and certificate re-key flows, then review how the associated certificate and private-key files are handled. Define whether the feature uses the proposed OpenSSL PKCS#12 generation, how the password is stored and reused, and where the generated .p12 is exposed; done means generation and re-key behavior are specified and verified.