feat(server): Backfill missing_fields on _nango_config

[nalanj]

Waiting on https://github.com/NangoHQ/nango/pull/3045 to land before this goes into review.

Fills out `missing_fields` as best as we can based on `_nango_config`. Unfortunately the `CUSTOM` `auth_method` won't be able to be backfilled, but all others were possible.

https://linear.app/nango/issue/NAN-2168/surface-integrationsconnections-errors-in-nango-ui

How I tested

• Run the migration against dev db with oauth and app auth_method integrations that don't have their settings filled
• Confirm in the database that the fields were correctly filled

[linear[bot]]

NAN-2168 Surface integrations/connections errors in Nango UI