[x] Improve and extend the existing ZeroTier backend schema using the OpenAPI specification for Self-hosted ZeroTier controllers (https://docs.zerotier.com/service/v1/).
coverage: 98.767% (+0.04%) from 98.728% when pulling 61b4cc69a7a9ac64f614318db9f53ca7d80d7c57 on issue-208/add-zerotier-backend into 96d811fe79b907fa6d8aef2378fc651c4d815179 on master.
Prerequisites:
Install Zerotier
Install Ztncui - Web Interface
Get your Zerotier Service API token
sudo cat /var/lib/zerotier-one/authtoken.secret
Tested with openwisp-controller
https://github.com/openwisp/netjsonconfig/assets/56113566/5787c6a8-1cd0-4d00-8d7c-a118252bed60
Todos
Closes #208