Add support for generating observations for unsupported nettests

[hellais]

Below I share the list of currently unsupported nettests. I group them into the ones for which we already have a deployed observation datamodel and those which might require a new observation table.

Works with existing observation tables:

• web_connectivity
• urlgetter
• browser_web
• facebook_messenger
• telegram
• whatsapp
• signal
• stunreachability
• tor
• http_invalid_request_line
• http_header_field_manipulation
• dnsping

Might require new observation tables:

• riseupvpn
• dash
• ndt
• openvpn
• psiphon
• torsf
• vanilla_tor
• dnscheck

You need to first implement a Model for the nettest parser: https://github.com/ooni/data/tree/main/oonidata/models and then a transform which converts it into relevant observation rows: https://github.com/ooni/data/tree/main/oonidata/transforms.

Use the make_* functions to convert the standard measurement keys into observations, for example see: https://github.com/ooni/data/blob/main/oonidata/transforms/nettests/telegram.py.