Add ability to filter features in layer according to current Map Theme

[blacktesta]

Feature description

Hi, developers and community! I ran into one problem. I wanted to filter data in my layers according to current map theme in QGIS and realised there is no way to do it. Provider feature filter is not a part of style, so it can't be changed by theme. Rule-based symbology can't get current theme because there is no @theme variable. That why i want to propose adding @theme or @currrent_theme variable in QGIS

Additional context

P.s. making filter part of style would also be great