HSTS support

**Is your feature request related to a problem? Please describe.**
I don't use authentik through reverse proxy, as it adds some extra complication, and it was easier to run it in a VM with its own IP. However, I no longer have control over HTTP vs HTTPS with this method. I would like to enforce HTTPS and disalow unencrypted traffic over standard HTTP, as it is less secure, but I haven't found a way to do so.

**Describe the solution you'd like**
The most user-friendly way could be a toggle in each Tennant/Brand to enforce HSTS. It could also be an environment variable set to `True`.

**Describe alternatives you've considered**
I could run authentik behind a reverse proxy again, which would allow for HSTS enforcement. However, as mentioned earlier, that requires more configuration than I am willing to do, especially since I use Nginx Proxy Manager instead of raw Nginx config files. Custom parameters are supported, but direct connections are far more simple.