Add more options for certificate sources

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

Currently, NPM supports Let's Encrypt or uploading custom certificates. As a self-hoster, all my services are internal, so Let's Encrypt doesn't work. Creating a custom certificate for each service is tedious. I wish there were better options in NPM for self-hosters!

Describe the solution you'd like

NPM should support configuring the ACME server, so that I can setup a Smallstep or Boulder CA server myself (e.g. direct it to https://my-ca.local for certificate requests)

Describe alternatives you've considered

• NPM generates certificates itself - and make it a simple option. NPM should be capable to do so and can offer the user to download the required certificates to install them on their client PC or other servers.
• NPM adds a pass-through option for proxying services (so I can use the service's own certificates)

Additional context

should become

The "several custom CAs" thing is just an idea, though practically most people probably only have one... as long as it remembers the info for that one I don't care if there's only one option :)

Also, the rename of "Custom" felt better with all the new options, but I have no strong feelings about it...

No files or tests are named in the issue. Start by locating the existing Let's Encrypt and custom-certificate configuration paths, then trace where certificate-source choices and ACME requests are handled. Done means users can configure a custom ACME server for certificate requests while retaining the existing certificate options.