Getting the `app_label` information on `post_migrate` signal

### Code of Conduct

- [x] I agree to follow Django's Code of Conduct

### Feature Description

`post_migrate` send the callback function an additional argument `app_label` that was passed to `manage.py` command line argument.

### Problem

I would like to get the `app_label` via the arguments come through the callback for `post_migrate`. I'm aware that `apps` argument has the whole list of installed apps. What I would like to get is the specific app on which the `migrate` command was executed.

Let's say I have multiple apps with models in my project. I would like to execute some callback functions on `post_migrate` signal, but only when the specific app was run, for example, if I run `python manage.py migrate` or `python manage.py migrate user`, then the callback function in my `user` app would run. In this case, it doesn't matter if `plan` argument is empty or not. When I run `python manage.py migrate blog` which does not depend on `user` app, then the callback function does not run.

I can check when the specific app is in the `plan` argument. For example, some migration depends on the app's model. However, when the plan is empty, I don't have any manner to check if the command was executed on a specific app.

### Request or proposal

request

### Additional Details

_No response_

### Implementation Suggestions

Adding an argument for `emit_post_migrate_signal` function, perhaps?

Start by tracing the post_migrate signal dispatch and the emit_post_migrate_signal function, then follow how the migrate command receives an app label from manage.py. Define how the callback should identify an explicitly selected app versus a full migration, and add coverage for empty plans and unrelated apps.

Written by the indexing model from the issue text.