Add an option to use full namespaces

Currently, Rider automatically unchecks the namespace provider option on the `Assets/Scripts` directory.
I personally find this fairly annoying because other editors(such as Zed and nvim) don't really have an option for this(at least that I know of).
Therefore, every other editor will use a full namespace, which makes more sense anyways, whilst Rider will complain about it.
Simply adding an option to Unity settings that would make the Scripts directory a namespace provider would be awesome.

Start by locating the Unity settings handling and the logic that treats Assets/Scripts as a namespace provider. Trace how Rider decides whether to enable that provider, then verify that a user-facing option can control it and that full namespaces are accepted without the current warning.

Written by the indexing model from the issue text.