Automatically use inner joins when filtering

Hi, first of all, thanks for building Cube.js and releasing it as open source, it's very cool software.

Is your feature request related to a problem? Please describe. If you filter on a field in a way that precludes it from being NULL, then there is no functional difference between an inner and a left join on tables in that field. However, there are slight performance benefits to having an inner join here.

Describe the solution you'd like When joins are added in order to support a filter, or really whenever a joined cube is filtered on, the generated join can be an INNER JOIN rather than LEFT JOIN. I think functionality that handles this case would be appreciated, in lieu of broader capability to influence join type and path.

This probably isn't super important / high priority, I'm just interested whether this in the roadmap. I can also have a go at doing it in a PR if somebody can direct me in the general direction of the relevant code.

Thanks again for your time and expertise!