Fix GET /alertmanager/{DatasourceId}/config/api/v1/alerts response

[papagian]

Grafana receiver GET responses should not expose the receiver secure settings.

This PR:

• renames GrafanaReceiver to PostableGrafanaReceiver (pointing to CreateAlertNotificationCommand) and introduces GettableGrafanaReceiver (pointing to dtos.AlertNotification)
• renames UserConfig to PostableUserConfig and introduces GettableUserConfig
• renames ApiAlertingConfig to PostableApiAlertingConfig and introduces GettableApiAlertingConfig
• renames ApiReceiver to PostableApiReceiver and introduces GettableApiReceiver
• renames GrafanaReceivers to PostableGrafanaReceivers and introduces GettableGrafanaReceivers

[owen-d]

I don't love the fact we need to add multiple representations for different actions or that we're duplicating validation code, but I suspect this will make it easier to hook into the existing codebase.