Support odk.ona.io as ODK server option

[spwoodcock]

Is your feature request related to a problem? Please describe.

• ONA host a free ODK instance, but it's behind their own Django wrapper and custom auth implementation.
• They also changed the API / endpoint schema from ODK, so requires some specific handling.
• The key thing is that this server is free and it would be a big win for users to support it in FMTM.
• This issue is more related to osm-fieldwork, but I have documented it here instead.

Describe the solution you'd like

Either:

1. Create a separate wrapper for accessing ONA and publishing forms via API token: OnaOdkCentralAsync.py or something.
2. Add extra logic to OdkCentralAsync.py to handle ONA server URLs, then change the logic / endpoints that are called to make it work.

• ONA provides API tokens in the user dashboard.
• We would need custom session handler to deal with the auth (as ONA uses their own auth endpoints).
• We should use their /v2 API as it's basically a wrapper of /v1 plus extra functionality: https://github.com/onaio/onadata/blob/main/onadata/apps/api/urls/v2_urls.py
• They seem to use a custom url format like /projects/1 and /forms/1 instead of ODK /projects/1/forms/1 (nested).