Add software updates settings schema

[nelsonkopliku]

Description

Initial PR supporting SUMA settings API.
Simply adding a schema where required information are going to be stored.

Consider that:

• software_update_settings is a one-row table like settings. Constraint added to the table
• password and ca_cert are store encrypted by leveraging https://github.com/danielberkompas/cloak_ecto

Implementation of module to get/apply suma settings will follow.

How was this tested?

Manually by running migration and manual insertions. More thorough testing will follow when implementing the context (service)

[dottorblaster]

LGTM with a minor nitpick