Add generate_letsencrypt_certificates script

[artonge]

Currently, there are only some information on the commands to run listed in the README. It would be easier to have a script to handle that automatically.

Sadly the testing procedure is complicated, so I have not made it. But I did test the code during my plugin review in august so it should be safe.

• Setup your TrueNAS VM and install the Nextcloud Plugin
• Make it accessible from the internet
• Point a domain name to its IP address
• Add the domain to TrueNAS and configure your Nextcloud Plugin to use the domain
• Enter the Nextcloud plugin shell and run:

  generate_letsencrypt_certificates <domain_name> <admin_email>