Managed Self-Signed CA cert

Is your feature request related to a problem? Please describe.
Currently, Nginx Proxy Manager (NPM) supports Let's Encrypt for publicly trusted certificates and allows uploading custom certificates. However, for internal networks, development environments, or scenarios where public CAs are not desired/feasible, managing self-signed certificates can be cumbersome. Users often need to manually generate a CA, sign server certificates, upload them, and manage renewals.

Describe the solution you'd like
I propose adding a new feature where NPM acts as its own internal Certificate Authority (CA). This would streamline the process of using self-signed certificates for managed hosts.

When adding a new Proxy Host and selecting a SSL cert, the user then can select the Managed CA, which will then create the necessary certificates automatically.

Describe alternatives you've considered
The primary alternatives involve manual management outside of NPM:

• Manually generating a root CA and server certificates using tools like OpenSSL or mkcert.
• Uploading these manually generated certificates as "Custom Certificates" within NPM.

These alternatives lack the integration and automation proposed by this feature, requiring more user effort and separate management workflows.

The issue names no files, tests, or entry points. Start by mapping how Proxy Hosts select SSL certificates and how Custom Certificates are managed, then define the CA, certificate generation, renewal, and trust workflow; done means Managed CA certificates can be selected for a Proxy Host and created automatically.