ImgUI UI should switch to DataModel::Provider instead of Accessors

[andy31415]

Using Accessorts ties things to ember and also prevents usage of AttributeAccessInterface.

See usage of Accessors in

https://github.com/project-chip/connectedhomeip/blob/master/examples/common/imgui_ui/windows

For example https://github.com/project-chip/connectedhomeip/blob/master/examples/common/imgui_ui/windows/light.cpp#L100

All Attributes Get and Set should be replaced with DataModel::Provider ReadAttribute and WriteAttribute. We likely want some shared code for most of this as we get/set common types.