Add group policy to configure shields default filterlists rather than URL specifics only

Platforms

Linux, macOS, Windows

Description

In https://github.com/brave/brave-core/pull/25710 we started to migrate towards group policies that match how Chromium does it. We currently have a few group policies for Brave specific features and this issue is to add a new policy that would allow a group policy admin to configure default filterlists themselves in shields as well. This may be useful for them to produce their own internal filterlist rule sets and turn them on by default or to configure a different default set of the predefined rules for administrative reasons.