Import a standard P7B certificate file to implement HTTPS on the management interface.

### Prerequisites

- [X] I have checked the [Wiki](https://github.com/AdguardTeam/AdGuardHome/wiki) and [Discussions](https://github.com/AdguardTeam/AdGuardHome/discussions) and found no answer

- [X] I have searched other issues and found no duplicates

- [X] I want to request a feature or enhancement and not ask a question

### Description

## Problem Description:
I have a commercially-issued, multi-host certificate, in the ubiquitous P7B format, that was acquired for a few internal devices, including my Adguard-Home server (Debian 11 host). I simply want to be able to upload and install it, in order to activate/utilise HTTPS on the management interface.

I HAVE LITERALLY NO INTEREST, USE OR CARE ABOUT USING THE CERTIFICATE FOR _**ANY**_ DNS FUNCTIONS!!!

Frustratingly, there does not appear to be any capacity whatsoever to perform/implement this extraordinarily rudimentary/common operation.

## Requested Feature:
Add functionality to facilitte simply selecting an appropriate P7B file on a (web) client computer via the management interface, then have it uploaded and installed on the server and activated for subsequent _web/management_ use, wholly, completely and utterly independent of any DNS considerations; incorporate system/service restart if required.

## Alternative Actions Taken:
None --> it's genuinely mind boggling to conceptualize that implementing such an exceptionally basic, and obvious, function has somehow been completely overlooked / forgotten to this point. Searching the web, the wiki, and discussions pages for a means to effect this operation has proven futile.