Enhancement:
This change introduces flexibility for users who manage package installations through other means and only want this role to join to a domain.

Reason:
Currently, the `ad_integration` role always attempts to install the required packages (like `realmd`, `sssd-ad`, `adcli`, etc.) listed in `__ad_integration_packages`. In environments where package management is handled externally (e.g., via configuration management outside of this role, pre-baked images, or immutable systems), this step can be redundant or even undesirable.

Result:
1. *New Variable:* Introduced a new boolean variable `ad_integration_manage_packages`.
2. *Default Value:* The variable defaults to `true` in `defaults/main.yml` to ensure backward compatibility. Existing playbooks using this role will continue to function as before without modification.
3. *Conditional Task:* Added a `when: ad_integration_manage_packages | bool` condition to the "Ensure required packages are installed" task in `tasks/main.yml`. The task will now only run if the flag is `true` (the default).
4. *Documentation:* Updated `README.md` to include the new `ad_integration_manage_packages` variable, explaining its purpose and default value.

Issue Tracker Tickets (Jira or BZ if any):