Add support for importing renewed local certificates

There is not currently a way to automate the renewal of locally imported certs. After I copy the updated cert and key to /etc/asterisk/keys and set appropriate ownership and permissions, I run fwconsole certificates import. But, I get an error:

No Certificates to import. Try placing a certificate (.crt) and its key (.crt) into /etc/asterisk/keys

It would be great if fwconsole certificates import just worked as expected (i.e. the cert is updated based on the new files provided).

I plan to give this a shot and submit a PR on github, but anyone is welcome to beat me to the punch.

See the discussion here: https://community.freepbx.org/t/how-to-renew-a-locally-imported-cert/50288