Add import of user's rules by URL

### Issue Details

In 4.0 there is no ability to import user rules (`ad blocking` / `DNS` rules).
Import by URL is useful for writing and debugging filter rules. On the desktop I prepared the rules, saved to gist or to my server, and on the mobile device I imported them and checked.
This significantly speeds up the process of creating rules.

### Proposed solution

* Add import by URL of
- ad blocking rules
- DNS filtering rules
Last used URL must be remembered.
Like in AdGuard for Android 3.6
* Add an option to overwrite or to append the contents of a user filter like in 3.6 in the import dialog.

### Alternative solution

_No response_