Feature Support: add suport for custom MitM CA

At present, AdGuard will generate a separate CA certificate for each installation, which does cause certain obstacles when using the local network HTTP proxy.

For example, since AdGuard is installed on a personal computer rather than a server, these devices may need to be installed with system updates and cannot be used for a long time. To switch the HTTP proxy to another device with AdGuard installed, the certificate must be reinstalled.

After replacing the computer, we also need to install new certificates for other devices in the local network, which is very troublesome for the TV boxes (such as Apple TV).

If you allow advanced users to customize the CA certificate, it helps to install a unified MitM CA on all devices without installing many CA certificates.