[Foundational Agents] Add database models for namespace and instance level settings

Add database migrations for top-level namespace and instance level settings. The following fields are necessary:

For top level namespace:

• foundational_agents_enabled_by_default: namespace, boolean
• foundational_agent_enabled: namespace, agent_reference, boolean

Consider using CascadingSettings for top level namespace so we can later add group/project settings

For instance level namespace:

• foundational_agents_enabled_by_default: boolean
• foundational_agent_enabled: agent_reference, boolean