Allow using oauth2 with Backend Application Flow / Client Credentials
Grant.

The use of a compute method on `oauth2_flow` when this field is touched by
the server environment mixin causes it to be defined twice as computed,
with differents settings, and this ultimately causes a warning message
in the logs:

```
WARNING odoo odoo.modules.registry: webservice.backend: inconsistent 'compute_sudo' for computed fields: protocol, url, auth_type, username, password, api_key, api_key_header, oauth2_flow, oauth2_clientid, oauth2_client_secret, oauth2_token_url, oauth2_authorization_url, oauth2_audience, oauth2_scope, content_type
```

We fix this by using an old fashioned onchange declaration on the
`auth_type` field.