* [ci] Added gsoc23 to CI build branches

* [feature] Added ZeroTier backend #208

Closes #208

* [changes] Extended ZeroTier backend schema

The schema is taken from OpenAPI specification:

https://docs.zerotier.com/service/v1/ (self-hosted controllers)
https://docs.zerotier.com/openapi/centralv1.json (central controllers)

The Zerotier network configuration keys taken from:

https://github.com/zerotier/ZeroTierOne/blob/dev/node/NetworkConfig.hpp

* [changes] Rendered ZeroTier configuration in JSON format

* [change] Included configuration suffix in the backend render

- Added propertyOrder to schema.

* [req-changes] Code improvements

- Removed redundant OpenAPI schema property 'example'.
- Changed backend render json indentation to 4 spaces.

* [req-changes] Added ZeroTier Parser

* [change] Removed redundant newline filtering before json.loads

- Added comment to `_generate_contents` method.

* [docs] Added ZeroTier backend page

* [req-changes] Improved ZeroTier parser

The ZeroTier parser should parse VPN
configurations with and without comments

* [tests] Added a few more comments for acid testing

---------

Co-authored-by: Federico Capoano <f.capoano@openwisp.io>

There are certain properties in the ZeroTier schema
that require default values. For example, if we don't provide
a default value for the `multicastLimit` property, it will be set to `0`
during an API call, which disables IPv4 communication for the network.

More information:
https://docs.zerotier.com/zerotier/rules

The schema is taken from OpenAPI specification:

https://docs.zerotier.com/service/v1/ (self-hosted controllers)
https://docs.zerotier.com/openapi/centralv1.json (central controllers)

The Zerotier network configuration keys taken from:

https://github.com/zerotier/ZeroTierOne/blob/dev/node/NetworkConfig.hpp

Closes #208

Co-authored-by: Federico Capoano <f.capoano@openwisp.io>

Closes #284