Validate Organization Settings against JSON schema

What does this MR do and why?

Related Issue #394801 (closed)

MR !123380 (merged) introduced JSONB as storage for the settings. Since we store settings as JSON, PostgreSQL is no longer aware of the data types being used. To address that, this MR adds validation of settings attribute against JSON schema.

We do not use these settings yet. A follow-up MR will start reading and writing the setting.

How to set up and validate locally

1. Using Rails console: Organizations::OrganizationSetting.new(settings: { some_key: 'invalid'}).valid? should be false
2. Organizations::OrganizationSetting.new(settings: { restricted_visibility_levels: [10]}).valid? should be true

MR acceptance checklist

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