[feature] Restrict access to sensitive fields for non-superuser

[pandafy]

Is your feature request related to a problem? Please describe.
Some shared objects in OpenWISP may contain sensitive information, such as passwords or private keys.

After #444, non-superusers will have read-only access to shared objects. Thus, they may get access to the sensitive information mention above which may cause security issues.

Describe the solution you'd like
Introduce reusable logic to flag specific model fields as sensitive.

Ensure that sensitive fields are automatically hidden in both the admin interface and the REST API when accessed by users who are not superusers.

[pandafy]

We would need to update the following models to use this feature:

• openwisp_controller.connection.models.Credentials
• openwisp_controller.pki.models.Ca
• openwisp_controller.pki.models.Cert