system: add backend call to renew self-signed web GUI certificate

[fichtner]

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• I have read the contributing guide lines at https://github.com/opnsense/core/blob/master/CONTRIBUTING.md

• I have searched the existing issues and I'm convinced that mine is new.

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

The web GUI does not automatically renew its own a self-signed certificate. See this (German) thread for example:

https://forum.opnsense.org/index.php?topic=4008.0

Describe the solution you'd like

A new backend call should be added to renew the certificate / issue a new one:

# configctl webgui restart renew

There are some edge cases to this (non-standard certificate, etc.) but for now this would help users to recover or to automate the process somewhat and we can build on it later on.

Also, during interface setup (setaddr.sh) we can ask for regeneration after making sure the user didn't want to revert to HTTP.

Describe alternatives you've considered

There is no alternative to eventually renewing a certificate, especially if it is expired or blocked by a browser. The web GUI may not be accessible for this reason so we need a console-based solution which can later be automated.

Additional context

Related to #805