Show available applications on user landing page

**Is your feature request related to a problem? Please describe.**
Not a problem, but an enhancement: I'd like to be able to use the user frontend as a kind of "App Directory".

**Describe the solution you'd like**
- Mark apps as "publicly available" in the admin backend
- Have the available apps listed on the user frontend after login
- Let users request access to an app via the frontend

Implementation steps/possible tasks:
- Add Option to "Show on landing page" to application properties in Admin backend
- (Optional) Scope to groups or something, so an app can be made "available" to only a subset of users. Might be a bit more complicated and maybe overkill, though.
- Show each available application in User frontend
- Add functionality to "request access" to the User frontend
- Create category "Requests" or something along those lines in Admin backend - Alternatively just send an email to an application admin/owner

Mockup:

**Describe alternatives you've considered**
Creating another webpage with a list and a form to request access, maybe populated by - in my case - terraform, which creates the authentik apps anyways.

**Additional context**
I think there already has been a feature request or issue about this _somewhere_, but I can't for the life of me find it. Last I looked at this was around March-ish, I think. So if there's already an issue about this, kindly let me know and close this as a duplicate.