Add `mdm.macos_migration` to app config endpoints

[gillespi314]

Related user story

#11534

Task

• Add new properties to config interface used with GET /config and PATCH /config endpoints.

Expected frontend TypeScript interface:

interface IConfig {
  org_info: {
    ...
    contact_url: string; // default is "https://fleetdm.com/company/contact"
  };
  ...
  mdm: {
    ...
    macos_migration: {
      enable: boolean; // default `false`
      mode: "voluntary" | "forced"; // default "voluntary"
      webhook_url: string; // default empty, but must be valid url if `enable: true` (subject to client-side validation)
    };
};

Expected fleetctl YAML spec:

apiVersion: v1
kind: config
spec:
  org_info:
    contact_url: https://fleetdm.com/company/contact
  mdm:
    macos_migration:
      enable: true
      mode: voluntary
      webhook_url: https://organization.com/send-request-here
  ...

[fleet-release]

MacOS migration blooms,
Config endpoints expand,
Glass city adapts.